refactor cmpIgnoreStyle and cmpIgnoreCase (#16399)

* init

* support strutils

* more

* better

* Call len once per string/cstring

* Change var to let

* Compare ternary on first char

* More appropriate param name

* fix

* better

* one test

* impl

* more efficient

* minor

Co-authored-by: Clyybber <darkmine956@gmail.com>

lib/std/private/strimpl.nim

@@ -0,0 +1,53 @@
+func toLowerAscii*(c: char): char {.inline.} =
+  if c in {'A'..'Z'}:
+    result = chr(ord(c) + (ord('a') - ord('A')))
+  else:
+    result = c
+
+template firstCharCaseSensitiveImpl(a, b: typed, aLen, bLen: int) =
+  if aLen == 0 or bLen == 0:
+    return aLen - bLen
+  if a[0] != b[0]: return ord(a[0]) - ord(b[0])
+
+template cmpIgnoreStyleImpl*(a, b: typed, firstCharCaseSensitive: static bool = false) =
+  # a, b are string or cstring
+  let aLen = a.len
+  let bLen = b.len
+  var i = 0
+  var j = 0
+  when firstCharCaseSensitive:
+    firstCharCaseSensitiveImpl(a, b, aLen, bLen)
+    inc i
+    inc j
+  while true:
+    while i < aLen and a[i] == '_': inc i
+    while j < bLen and b[j] == '_': inc j
+    let aa = if i < aLen: toLowerAscii(a[i]) else: '\0'
+    let bb = if j < bLen: toLowerAscii(b[j]) else: '\0'
+    result = ord(aa) - ord(bb)
+    if result != 0: return result
+    # the characters are identical:
+    if i >= aLen:
+      # both cursors at the end:
+      if j >= bLen: return 0
+      # not yet at the end of 'b':
+      return -1
+    elif j >= bLen:
+      return 1
+    inc i
+    inc j
+
+template cmpIgnoreCaseImpl*(a, b: typed, firstCharCaseSensitive: static bool = false) =
+  # a, b are string or cstring
+  let aLen = a.len
+  let bLen = b.len
+  var i = 0
+  when firstCharCaseSensitive:
+    firstCharCaseSensitiveImpl(a, b, aLen, bLen)
+    inc i
+  var m = min(aLen, bLen)
+  while i < m:
+    result = ord(toLowerAscii(a[i])) - ord(toLowerAscii(b[i]))
+    if result != 0: return
+    inc i
+  result = aLen - bLen