1 files changed, 76 insertions, 0 deletions
diff --git a/2020/eighteen.nim b/2020/eighteen.nim
new file mode 100644
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+# Day Eighteen: Operation Order
+import std/[os, strutils]
+
+let input = paramStr(1).readFile().strip().split("\n")
+
+# cheese: add parenthesis and eval
+
+func evaluate(a, b: int, op: char): int =
+  if op == '+':
+    return a + b
+  elif op == '*':
+    return a * b
+  else:
+    debugEcho "this should never happen."
+    return -1
+
+var sum = 0
+for line in input:
+  var stack: seq[tuple[val: int, op: char]] = @[(0, '+')]
+  for c in line:
+    let last = stack.len - 1
+    if c == ' ':
+      discard
+    elif c == '+' or c == '*':
+      stack[last].op = c
+    elif c == '(':
+      stack.add((0, '+'))
+    elif c == ')':
+      stack[last-1] = (evaluate(stack[last-1].val, stack.pop.val, stack[last-1].op), '+')
+    else:
+      stack[last] = (evaluate(stack[last].val, parseInt($c), stack[last].op), ' ')
+  sum += stack[stack.len - 1].val
+echo sum
+
+func shunting(expression: string): string =
+  var stack: seq[char]
+  for t in expression:
+    if t == ' ':
+      discard
+    elif t == '+':
+      while stack.len > 0 and stack[stack.len-1] == '+':
+        result.add(stack.pop)
+      stack.add(t)
+    elif t == '*':
+      while stack.len > 0 and (stack[stack.len-1] == '+' or stack[stack.len-1] == '*'):
+        result.add(stack.pop)
+      stack.add(t)
+    elif t == '(':
+      stack.add(t)
+    elif t == ')':
+      while stack.len > 0 and stack[stack.len - 1] != '(':
+        result.add(stack.pop)
+      assert stack.pop == '('
+    else:
+      result.add(t)
+  while stack.len != 0:
+    result.add(stack.pop)
+
+func evaluate(expression: string): int =
+  var stack: seq[int]
+  for s in expression:
+    if s == ' ':
+      discard
+    elif s == '+':
+      stack.add(stack.pop + stack.pop)
+    elif s == '*':
+      stack.add(stack.pop * stack.pop)
+    else:
+      stack.add(parseInt($s))
+  assert stack.len == 1
+  return stack[0]
+
+sum = 0
+for line in input:
+  sum += evaluate(line.shunting())
+echo sum