Significantly improve performance of `:reload` (#4457)

* bump ropey to 1.5.1-alpha

* significantly improve performance of :reload

4 files changed, 389 insertions, 126 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 1d0ee4df..05a0396f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -27,9 +27,9 @@ dependencies = [
 
 [[package]]
 name = "aho-corasick"
-version = "0.7.18"
+version = "0.7.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1e37cfd5e7657ada45f742d6e99ca5788580b5c529dc78faf11ece6dc702656f"
+checksum = "cc936419f96fa211c1b9166887b38e5e40b19958e5b895be7c1f93adec7071ac"
 dependencies = [
  "memchr",
 ]
@@ -80,9 +80,9 @@ dependencies = [
 
 [[package]]
 name = "bumpalo"
-version = "3.11.0"
+version = "3.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c1ad822118d20d2c234f427000d5acc36eabe1e29a348c89b63dd60b13f28e5d"
+checksum = "572f695136211188308f16ad2ca5c851a712c464060ae6974944458eb83880ba"
 
 [[package]]
 name = "bytecount"
@@ -92,9 +92,9 @@ checksum = "2c676a478f63e9fa2dd5368a42f28bba0d6c560b775f38583c8bbaa7fcd67c9c"
 
 [[package]]
 name = "bytes"
-version = "1.2.1"
+version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ec8a7b6a70fde80372154c65702f00a0f56f3e1c36abbc6c440484be248856db"
+checksum = "dfb24e866b15a1af2a1b663f10c6b6b8f397a84aadb828f12e5b289ec23a3a3c"
 
 [[package]]
 name = "cassowary"
@@ -149,6 +149,16 @@ dependencies = [
 ]
 
 [[package]]
+name = "codespan-reporting"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3538270d33cc669650c4b093848450d380def10c331d38c768e34cac80576e6e"
+dependencies = [
+ "termcolor",
+ "unicode-width",
+]
+
+[[package]]
 name = "content_inspector"
 version = "0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -165,12 +175,11 @@ checksum = "5827cebf4670468b8772dd191856768aedcb1b0278a04f989f7766351917b9dc"
 
 [[package]]
 name = "crossbeam-utils"
-version = "0.8.11"
+version = "0.8.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "51887d4adc7b564537b15adcfb307936f8075dfcd5f00dde9a9f1d29383682bc"
+checksum = "4fb766fa798726286dbbb842f174001dab8abc7b627a1dd86e0b7222a95d929f"
 dependencies = [
  "cfg-if",
- "once_cell",
 ]
 
 [[package]]
@@ -200,6 +209,50 @@ dependencies = [
 ]
 
 [[package]]
+name = "cxx"
+version = "1.0.82"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d4a41a86530d0fe7f5d9ea779916b7cadd2d4f9add748b99c2c029cbbdfaf453"
+dependencies = [
+ "cc",
+ "cxxbridge-flags",
+ "cxxbridge-macro",
+ "link-cplusplus",
+]
+
+[[package]]
+name = "cxx-build"
+version = "1.0.82"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06416d667ff3e3ad2df1cd8cd8afae5da26cf9cec4d0825040f88b5ca659a2f0"
+dependencies = [
+ "cc",
+ "codespan-reporting",
+ "once_cell",
+ "proc-macro2",
+ "quote",
+ "scratch",
+ "syn",
+]
+
+[[package]]
+name = "cxxbridge-flags"
+version = "1.0.82"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "820a9a2af1669deeef27cb271f476ffd196a2c4b6731336011e0ba63e2c7cf71"
+
+[[package]]
+name = "cxxbridge-macro"
+version = "1.0.82"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a08a6e2fcc370a089ad3b4aaf54db3b1b4cee38ddabce5896b33eb693275f470"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
 name = "dirs-next"
 version = "2.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -345,9 +398,9 @@ dependencies = [
 
 [[package]]
 name = "getrandom"
-version = "0.2.7"
+version = "0.2.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4eb1a864a501629691edf6c15a593b7a51eebaa1e8468e9ddc623de7c9b58ec6"
+checksum = "c05aeb6a22b8f62540c194aac980f2115af067bfe15a0734d7277a768d396b31"
 dependencies = [
  "cfg-if",
  "libc",
@@ -436,6 +489,7 @@ dependencies = [
  "etcetera",
  "hashbrown 0.13.1",
  "helix-loader",
+ "imara-diff",
  "log",
  "once_cell",
  "quickcheck",
@@ -443,7 +497,6 @@ dependencies = [
  "ropey",
  "serde",
  "serde_json",
- "similar",
  "slotmap",
  "smallvec",
  "smartstring",
@@ -594,19 +647,29 @@ dependencies = [
 
 [[package]]
 name = "iana-time-zone"
-version = "0.1.47"
+version = "0.1.53"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4c495f162af0bf17656d0014a0eded5f3cd2f365fdd204548c2869db89359dc7"
+checksum = "64c122667b287044802d6ce17ee2ddf13207ed924c712de9a66a5814d5b64765"
 dependencies = [
  "android_system_properties",
  "core-foundation-sys",
+ "iana-time-zone-haiku",
  "js-sys",
- "once_cell",
  "wasm-bindgen",
  "winapi",
 ]
 
 [[package]]
+name = "iana-time-zone-haiku"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0703ae284fc167426161c2e3f1da3ea71d94b21bedbcc9494e92b28e334e3dca"
+dependencies = [
+ "cxx",
+ "cxx-build",
+]
+
+[[package]]
 name = "idna"
 version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -635,6 +698,16 @@ dependencies = [
 ]
 
 [[package]]
+name = "imara-diff"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e98c1d0ad70fc91b8b9654b1f33db55e59579d3b3de2bffdced0fdb810570cb8"
+dependencies = [
+ "ahash 0.8.2",
+ "hashbrown 0.12.3",
+]
+
+[[package]]
 name = "indoc"
 version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -651,15 +724,15 @@ dependencies = [
 
 [[package]]
 name = "itoa"
-version = "1.0.3"
+version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6c8af84674fe1f223a982c933a0ee1086ac4d4052aa0fb8060c12c6ad838e754"
+checksum = "4217ad341ebadf8d8e724e264f13e593e0648f5b3e94b3896a5df283be015ecc"
 
 [[package]]
 name = "js-sys"
-version = "0.3.59"
+version = "0.3.60"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "258451ab10b34f8af53416d1fdab72c22e805f0c92a1136d59470ec0b11138b2"
+checksum = "49409df3e3bf0856b916e2ceaca09ee28e6871cf7d9ce97a692cacfdb2a25a47"
 dependencies = [
  "wasm-bindgen",
 ]
@@ -672,9 +745,9 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
 
 [[package]]
 name = "libc"
-version = "0.2.132"
+version = "0.2.137"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8371e4e5341c3a96db127eb2465ac681ced4c433e01dd0e938adbef26ba93ba5"
+checksum = "fc7fcc620a3bff7cdd7a365be3376c97191aeaccc2a603e600951e452615bf89"
 
 [[package]]
 name = "libloading"
@@ -687,10 +760,19 @@ dependencies = [
 ]
 
 [[package]]
+name = "link-cplusplus"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9272ab7b96c9046fbc5bc56c06c117cb639fe2d509df0c421cad82d2915cf369"
+dependencies = [
+ "cc",
+]
+
+[[package]]
 name = "lock_api"
-version = "0.4.8"
+version = "0.4.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9f80bf5aacaf25cbfc8210d1cfb718f2bf3b11c4c54e5afe36c236853a8ec390"
+checksum = "435011366fe56583b16cf956f9df0095b405b82d76425bc8981c0e22e60ec4df"
 dependencies = [
  "autocfg",
  "scopeguard",
@@ -726,18 +808,18 @@ checksum = "2dffe52ecf27772e601905b7522cb4ef790d2cc203488bbd0e2fe85fcb74566d"
 
 [[package]]
 name = "memmap2"
-version = "0.5.7"
+version = "0.5.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "95af15f345b17af2efc8ead6080fb8bc376f8cec1b35277b935637595fe77498"
+checksum = "4b182332558b18d807c4ce1ca8ca983b34c3ee32765e47b3f0f69b90355cc1dc"
 dependencies = [
  "libc",
 ]
 
 [[package]]
 name = "mio"
-version = "0.8.4"
+version = "0.8.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "57ee1c23c7c63b0c9250c339ffdc69255f110b298b901b9f6c82547b7b87caaf"
+checksum = "e5d732bc30207a6423068df043e3d02e0735b155ad7ce1a6f76fe2baa5b158de"
 dependencies = [
  "libc",
  "log",
@@ -766,9 +848,9 @@ dependencies = [
 
 [[package]]
 name = "num_cpus"
-version = "1.13.1"
+version = "1.14.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "19e64526ebdee182341572e50e9ad03965aa510cd94427a4549448f285e957a1"
+checksum = "f6058e64324c71e02bc2b150e4f3bc8286db6c83092132ffa3f6b1eab0f9def5"
 dependencies = [
  "hermit-abi",
  "libc",
@@ -792,9 +874,9 @@ dependencies = [
 
 [[package]]
 name = "parking_lot_core"
-version = "0.9.3"
+version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "09a279cbf25cb0757810394fbc1e359949b59e348145c643a939a525692e6929"
+checksum = "4dc9e0dc2adc1c69d09143aff38d3d30c5c3f0df0dad82e6d25547af174ebec0"
 dependencies = [
  "cfg-if",
  "libc",
@@ -823,9 +905,9 @@ checksum = "8b870d8c151b6f2fb93e84a13146138f05d02ed11c7e7c54f8826aaaf7c9f184"
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.43"
+version = "1.0.47"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0a2ca2c61bc9f3d74d2886294ab7b9853abd9c1ad903a3ac7815c58989bb7bab"
+checksum = "5ea3d908b0e36316caf9e9e2c4625cdde190a7e6f440d794667ed17a1855e725"
 dependencies = [
  "unicode-ident",
 ]
@@ -870,9 +952,9 @@ dependencies = [
 
 [[package]]
 name = "rand_core"
-version = "0.6.3"
+version = "0.6.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d34f1408f55294453790c48b2f1ebbb1c5b4b7563eb1f418bcfcfdbb06ebb4e7"
+checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
 dependencies = [
  "getrandom",
 ]
@@ -916,9 +998,9 @@ checksum = "6c230d73fb8d8c1b9c0b3135c5142a8acee3a0558fb8db5cf1cb65f8d7862132"
 
 [[package]]
 name = "regex-syntax"
-version = "0.6.27"
+version = "0.6.28"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a3f87b73ce11b1619a3c6332f45341e0047173771e8b8b73f87bfeefb7b56244"
+checksum = "456c603be3e8d448b072f410900c09faf164fbce2d480456f50eea6e25f9c848"
 
 [[package]]
 name = "remove_dir_all"
@@ -961,19 +1043,25 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d29ab0c6d3fc0ee92fe66e2d99f700eab17a8d57d1c1d3b748380fb20baa78cd"
 
 [[package]]
+name = "scratch"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8132065adcfd6e02db789d9285a0deb2f3fcb04002865ab67d5fb103533898"
+
+[[package]]
 name = "serde"
-version = "1.0.147"
+version = "1.0.148"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d193d69bae983fc11a79df82342761dfbf28a99fc8d203dca4c3c1b590948965"
+checksum = "e53f64bb4ba0191d6d0676e1b141ca55047d83b74f5607e6d8eb88126c52c2dc"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.147"
+version = "1.0.148"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4f1d362ca8fc9c3e3a7484440752472d68a6caa98f1ab81d99b5dfe517cec852"
+checksum = "a55492425aa53521babf6137309e7d34c20bbfbbfcfe2c7f3a047fd1f6b92c0c"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -982,9 +1070,9 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.88"
+version = "1.0.89"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8e8b3801309262e8184d9687fb697586833e939767aea0dda89f5a8e650e8bd7"
+checksum = "020ff22c755c2ed3f8cf162dbb41a7268d934702f3ed3631656ea597e08fc3db"
 dependencies = [
  "itoa",
  "ryu",
@@ -1045,12 +1133,6 @@ dependencies = [
 ]
 
 [[package]]
-name = "similar"
-version = "2.2.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "420acb44afdae038210c99e69aae24109f32f15500aa708e81d46c9f29d55fcf"
-
-[[package]]
 name = "slab"
 version = "0.4.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1121,9 +1203,9 @@ checksum = "9d9199fa80c817e074620be84374a520062ebac833f358d74b37060ce4a0f2c0"
 
 [[package]]
 name = "syn"
-version = "1.0.99"
+version = "1.0.104"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "58dbef6ec655055e20b86b15a8cc6d439cca19b667537ac6a1369572d151ab13"
+checksum = "4ae548ec36cf198c0ef7710d3c230987c2d6d7bd98ad6edc0274462724c585ce"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1145,6 +1227,15 @@ dependencies = [
 ]
 
 [[package]]
+name = "termcolor"
+version = "1.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bab24d30b911b2376f3a13cc2cd443142f0c81dda04c118693e35b3835757755"
+dependencies = [
+ "winapi-util",
+]
+
+[[package]]
 name = "termini"
 version = "0.1.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1301,9 +1392,9 @@ checksum = "2281c8c1d221438e373249e065ca4989c4c36952c211ff21a0ee91c44a3869e7"
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.3"
+version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c4f5b37a154999a8f3f98cc23a628d850e154479cd94decf3414696e12e31aaf"
+checksum = "6ceab39d59e4c9499d4e5a8ee0e2735b891bb7308ac83dfb4e80cad195c9f6f3"
 
 [[package]]
 name = "unicode-linebreak"
@@ -1317,9 +1408,9 @@ dependencies = [
 
 [[package]]
 name = "unicode-normalization"
-version = "0.1.21"
+version = "0.1.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "854cbdc4f7bc6ae19c820d44abdc3277ac3e1b2b93db20a636825d9322fb60e6"
+checksum = "5c5713f0fc4b5db668a2ac63cdb7bb4469d8c9fed047b1d0292cc7b0ce2ba921"
 dependencies = [
  "tinyvec",
 ]
@@ -1373,9 +1464,9 @@ checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
 
 [[package]]
 name = "wasm-bindgen"
-version = "0.2.82"
+version = "0.2.83"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fc7652e3f6c4706c8d9cd54832c4a4ccb9b5336e2c3bd154d5cccfbf1c1f5f7d"
+checksum = "eaf9f5aceeec8be17c128b2e93e031fb8a4d469bb9c4ae2d7dc1888b26887268"
 dependencies = [
  "cfg-if",
  "wasm-bindgen-macro",
@@ -1383,9 +1474,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-backend"
-version = "0.2.82"
+version = "0.2.83"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "662cd44805586bd52971b9586b1df85cdbbd9112e4ef4d8f41559c334dc6ac3f"
+checksum = "4c8ffb332579b0557b52d268b91feab8df3615f265d5270fec2a8c95b17c1142"
 dependencies = [
  "bumpalo",
  "log",
@@ -1398,9 +1489,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-macro"
-version = "0.2.82"
+version = "0.2.83"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b260f13d3012071dfb1512849c033b1925038373aea48ced3012c09df952c602"
+checksum = "052be0f94026e6cbc75cdefc9bae13fd6052cdcaf532fa6c45e7ae33a1e6c810"
 dependencies = [
  "quote",
  "wasm-bindgen-macro-support",
@@ -1408,9 +1499,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-macro-support"
-version = "0.2.82"
+version = "0.2.83"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5be8e654bdd9b79216c2929ab90721aa82faf65c48cdf08bdc4e7f51357b80da"
+checksum = "07bc0c051dc5f23e307b13285f9d75df86bfdf816c5721e573dec1f9b8aa193c"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1421,9 +1512,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-bindgen-shared"
-version = "0.2.82"
+version = "0.2.83"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6598dd0bd3c7d51095ff6531a5b23e02acdc81804e30d8f07afb77b7215a140a"
+checksum = "1c38c045535d93ec4f0b4defec448e4291638ee608530863b1e2ba115d4fff7f"
 
 [[package]]
 name = "which"
@@ -1469,46 +1560,60 @@ checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
 
 [[package]]
 name = "windows-sys"
-version = "0.36.1"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ea04155a16a59f9eab786fe12a4a450e75cdb175f9e0d80da1e17db09f55b8d2"
+checksum = "5a3e1820f08b8513f676f7ab6c1f99ff312fb97b553d30ff4dd86f9f15728aa7"
 dependencies = [
+ "windows_aarch64_gnullvm",
  "windows_aarch64_msvc",
  "windows_i686_gnu",
  "windows_i686_msvc",
  "windows_x86_64_gnu",
+ "windows_x86_64_gnullvm",
  "windows_x86_64_msvc",
 ]
 
 [[package]]
+name = "windows_aarch64_gnullvm"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41d2aa71f6f0cbe00ae5167d90ef3cfe66527d6f613ca78ac8024c3ccab9a19e"
+
+[[package]]
 name = "windows_aarch64_msvc"
-version = "0.36.1"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9bb8c3fd39ade2d67e9874ac4f3db21f0d710bee00fe7cab16949ec184eeaa47"
+checksum = "dd0f252f5a35cac83d6311b2e795981f5ee6e67eb1f9a7f64eb4500fbc4dcdb4"
 
 [[package]]
 name = "windows_i686_gnu"
-version = "0.36.1"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "180e6ccf01daf4c426b846dfc66db1fc518f074baa793aa7d9b9aaeffad6a3b6"
+checksum = "fbeae19f6716841636c28d695375df17562ca208b2b7d0dc47635a50ae6c5de7"
 
 [[package]]
 name = "windows_i686_msvc"
-version = "0.36.1"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e2e7917148b2812d1eeafaeb22a97e4813dfa60a3f8f78ebe204bcc88f12f024"
+checksum = "84c12f65daa39dd2babe6e442988fc329d6243fdce47d7d2d155b8d874862246"
 
 [[package]]
 name = "windows_x86_64_gnu"
-version = "0.36.1"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf7b1b21b5362cbc318f686150e5bcea75ecedc74dd157d874d754a2ca44b0ed"
+
+[[package]]
+name = "windows_x86_64_gnullvm"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4dcd171b8776c41b97521e5da127a2d86ad280114807d0b2ab1e462bc764d9e1"
+checksum = "09d525d2ba30eeb3297665bd434a54297e4170c7f1a44cad4ef58095b4cd2028"
 
 [[package]]
 name = "windows_x86_64_msvc"
-version = "0.36.1"
+version = "0.42.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c811ca4a8c853ef420abd8592ba53ddbbac90410fab6903b3e79972a631f7680"
+checksum = "f40009d85759725a34da6d89a94e63d7bdc50a862acf0dbc7c8e488f1edcb6f5"
 
 [[package]]
 name = "xtask"
diff --git a/helix-core/Cargo.toml b/helix-core/Cargo.toml
index 09665b91..31b6546f 100644
--- a/helix-core/Cargo.toml
+++ b/helix-core/Cargo.toml
@@ -38,7 +38,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 toml = "0.5"
 
-similar = "2.2"
+imara-diff = "0.1.0"
 
 encoding_rs = "0.8"
 
diff --git a/helix-core/src/diff.rs b/helix-core/src/diff.rs
index 6960c679..c754da30 100644
--- a/helix-core/src/diff.rs
+++ b/helix-core/src/diff.rs
@@ -1,58 +1,195 @@
-use crate::{Rope, Transaction};
+use std::ops::Range;
+use std::time::Instant;
 
-/// Compares `old` and `new` to generate a [`Transaction`] describing
-/// the steps required to get from `old` to `new`.
-pub fn compare_ropes(old: &Rope, new: &Rope) -> Transaction {
-    // `similar` only works on contiguous data, so a `Rope` has
-    // to be temporarily converted into a `String`.
-    let old_converted = old.to_string();
-    let new_converted = new.to_string();
-
-    // A timeout is set so after 1 seconds, the algorithm will start
-    // approximating. This is especially important for big `Rope`s or
-    // `Rope`s that are extremely dissimilar to each other.
-    let mut config = similar::TextDiff::configure();
-    config.timeout(std::time::Duration::from_secs(1));
-
-    let diff = config.diff_chars(&old_converted, &new_converted);
-
-    // The current position of the change needs to be tracked to
-    // construct the `Change`s.
-    let mut pos = 0;
-    Transaction::change(
-        old,
-        diff.ops()
+use imara_diff::intern::InternedInput;
+use imara_diff::Algorithm;
+use ropey::RopeSlice;
+
+use crate::{ChangeSet, Rope, Tendril, Transaction};
+
+/// A `imara_diff::Sink` that builds a `ChangeSet` for a character diff of a hunk
+struct CharChangeSetBuilder<'a> {
+    res: &'a mut ChangeSet,
+    hunk: &'a InternedInput<char>,
+    pos: u32,
+}
+
+impl imara_diff::Sink for CharChangeSetBuilder<'_> {
+    type Out = ();
+    fn process_change(&mut self, before: Range<u32>, after: Range<u32>) {
+        self.res.retain((before.start - self.pos) as usize);
+        self.res.delete(before.len());
+        self.pos = before.end;
+
+        let res = self.hunk.after[after.start as usize..after.end as usize]
+            .iter()
+            .map(|&token| self.hunk.interner[token])
+            .collect();
+
+        self.res.insert(res);
+    }
+
+    fn finish(self) -> Self::Out {
+        self.res.retain(self.hunk.before.len() - self.pos as usize);
+    }
+}
+
+struct LineChangeSetBuilder<'a> {
+    res: ChangeSet,
+    after: RopeSlice<'a>,
+    file: &'a InternedInput<RopeSlice<'a>>,
+    current_hunk: InternedInput<char>,
+    pos: u32,
+}
+
+impl imara_diff::Sink for LineChangeSetBuilder<'_> {
+    type Out = ChangeSet;
+
+    fn process_change(&mut self, before: Range<u32>, after: Range<u32>) {
+        let len = self.file.before[self.pos as usize..before.start as usize]
             .iter()
-            .map(|op| op.as_tag_tuple())
-            .filter_map(|(tag, old_range, new_range)| {
-                // `old_pos..pos` is equivalent to `start..end` for where
-                // the change should be applied.
-                let old_pos = pos;
-                pos += old_range.end - old_range.start;
-
-                match tag {
-                    // Semantically, inserts and replacements are the same thing.
-                    similar::DiffTag::Insert | similar::DiffTag::Replace => {
-                        // This is the text from the `new` rope that should be
-                        // inserted into `old`.
-                        let text: &str = {
-                            let start = new.char_to_byte(new_range.start);
-                            let end = new.char_to_byte(new_range.end);
-                            &new_converted[start..end]
-                        };
-                        Some((old_pos, pos, Some(text.into())))
+            .map(|&it| self.file.interner[it].len_chars())
+            .sum();
+        self.res.retain(len);
+        self.pos = before.end;
+
+        // do not perform diffs on large hunks
+        let len_before = before.end - before.start;
+        let len_after = after.end - after.start;
+
+        // Pure insertions/removals do not require a character diff.
+        // Very large changes are ignored because their character diff is expensive to compute
+        // TODO adjust heuristic to detect large changes?
+        if len_before == 0
+            || len_after == 0
+            || len_after > 5 * len_before
+            || 5 * len_after < len_before && len_before > 10
+            || len_before + len_after > 200
+        {
+            let remove = self.file.before[before.start as usize..before.end as usize]
+                .iter()
+                .map(|&it| self.file.interner[it].len_chars())
+                .sum();
+            self.res.delete(remove);
+            let mut fragment = Tendril::new();
+            if len_after > 500 {
+                // copying a rope line by line is slower then copying the entire
+                // rope. Use to_string for very large changes instead..
+                if self.file.after.len() == after.end as usize {
+                    if after.start == 0 {
+                        fragment = self.after.to_string().into();
+                    } else {
+                        let start = self.after.line_to_char(after.start as usize);
+                        fragment = self.after.slice(start..).to_string().into();
                     }
-                    similar::DiffTag::Delete => Some((old_pos, pos, None)),
-                    similar::DiffTag::Equal => None,
+                } else if after.start == 0 {
+                    let end = self.after.line_to_char(after.end as usize);
+                    fragment = self.after.slice(..end).to_string().into();
+                } else {
+                    let start = self.after.line_to_char(after.start as usize);
+                    let end = self.after.line_to_char(after.end as usize);
+                    fragment = self.after.slice(start..end).to_string().into();
                 }
-            }),
-    )
+            } else {
+                for &line in &self.file.after[after.start as usize..after.end as usize] {
+                    for chunk in self.file.interner[line].chunks() {
+                        fragment.push_str(chunk)
+                    }
+                }
+            };
+            self.res.insert(fragment);
+        } else {
+            // for reasonably small hunks, generating a ChangeSet from char diff can save memory
+            // TODO use a tokenizer (word diff?) for improved performance
+            let hunk_before = self.file.before[before.start as usize..before.end as usize]
+                .iter()
+                .flat_map(|&it| self.file.interner[it].chars());
+            let hunk_after = self.file.after[after.start as usize..after.end as usize]
+                .iter()
+                .flat_map(|&it| self.file.interner[it].chars());
+            self.current_hunk.update_before(hunk_before);
+            self.current_hunk.update_after(hunk_after);
+
+            // the histogram heuristic does not work as well
+            // for characters because the same characters often reoccur
+            // use myer diff instead
+            imara_diff::diff(
+                Algorithm::Myers,
+                &self.current_hunk,
+                CharChangeSetBuilder {
+                    res: &mut self.res,
+                    hunk: &self.current_hunk,
+                    pos: 0,
+                },
+            );
+
+            self.current_hunk.clear();
+        }
+    }
+
+    fn finish(mut self) -> Self::Out {
+        let len = self.file.before[self.pos as usize..]
+            .iter()
+            .map(|&it| self.file.interner[it].len_chars())
+            .sum();
+
+        self.res.retain(len);
+        self.res
+    }
+}
+
+struct RopeLines<'a>(RopeSlice<'a>);
+
+impl<'a> imara_diff::intern::TokenSource for RopeLines<'a> {
+    type Token = RopeSlice<'a>;
+    // TODO: improve performance of lines iterator (https://github.com/cessen/ropey/issues/25)
+    type Tokenizer = ropey::iter::Lines<'a>;
+
+    fn tokenize(&self) -> Self::Tokenizer {
+        self.0.lines()
+    }
+
+    fn estimate_tokens(&self) -> u32 {
+        // we can provide a perfect estimate which is very nice for performance
+        self.0.len_lines() as u32
+    }
+}
+
+/// Compares `old` and `new` to generate a [`Transaction`] describing
+/// the steps required to get from `old` to `new`.
+pub fn compare_ropes(before: &Rope, after: &Rope) -> Transaction {
+    let start = Instant::now();
+    let res = ChangeSet::with_capacity(32);
+    let after = after.slice(..);
+    let file = InternedInput::new(RopeLines(before.slice(..)), RopeLines(after));
+    let builder = LineChangeSetBuilder {
+        res,
+        file: &file,
+        after,
+        pos: 0,
+        current_hunk: InternedInput::default(),
+    };
+
+    let res = imara_diff::diff(Algorithm::Histogram, &file, builder).into();
+
+    log::debug!(
+        "rope diff took {}s",
+        Instant::now().duration_since(start).as_secs_f64()
+    );
+    res
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    fn test_identity(a: &str, b: &str) {
+        let mut old = Rope::from(a);
+        let new = Rope::from(b);
+        compare_ropes(&old, &new).apply(&mut old);
+        assert_eq!(old, new);
+    }
+
     quickcheck::quickcheck! {
         fn test_compare_ropes(a: String, b: String) -> bool {
             let mut old = Rope::from(a);
@@ -61,4 +198,25 @@ mod tests {
             old == new
         }
     }
+
+    #[test]
+    fn equal_files() {
+        test_identity("foo", "foo");
+    }
+
+    #[test]
+    fn trailing_newline() {
+        test_identity("foo\n", "foo");
+        test_identity("foo", "foo\n");
+    }
+
+    #[test]
+    fn new_file() {
+        test_identity("", "foo");
+    }
+
+    #[test]
+    fn deleted_file() {
+        test_identity("foo", "");
+    }
 }
diff --git a/helix-core/src/transaction.rs b/helix-core/src/transaction.rs
index 3fb39413..482fd6d9 100644
--- a/helix-core/src/transaction.rs
+++ b/helix-core/src/transaction.rs
@@ -56,7 +56,7 @@ impl ChangeSet {
     }
 
     // Changeset builder operations: delete/insert/retain
-    fn delete(&mut self, n: usize) {
+    pub(crate) fn delete(&mut self, n: usize) {
         use Operation::*;
         if n == 0 {
             return;
@@ -71,7 +71,7 @@ impl ChangeSet {
         }
     }
 
-    fn insert(&mut self, fragment: Tendril) {
+    pub(crate) fn insert(&mut self, fragment: Tendril) {
         use Operation::*;
 
         if fragment.is_empty() {
@@ -93,7 +93,7 @@ impl ChangeSet {
         self.changes.push(new_last);
     }
 
-    fn retain(&mut self, n: usize) {
+    pub(crate) fn retain(&mut self, n: usize) {
         use Operation::*;
         if n == 0 {
             return;