From ef3ed45c1240197b721f38a01ec48e7dc6e7c179 Mon Sep 17 00:00:00 2001
From: adisbladis <adisbladis@gmail.com>
Date: Mon, 7 Dec 2020 13:42:59 +0100
Subject: [PATCH] nix-gitignore: Optimise performance

There are a few operations in this library that naively runs on every
iteration while they could be cached.

For a simple test repository with a small number of files and ~1000
gitignore patterns this brings memory usage down from ~233M to ~157M
and wall time from 2.6s down to 0.78s.

This should scale similarly with the number of files in a repository.
---
 pkgs/build-support/nix-gitignore/default.nix | 26 +++++++++++---------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/pkgs/build-support/nix-gitignore/default.nix b/pkgs/build-support/nix-gitignore/default.nix
index 28ee6bad5540..fba09adc2d41 100644
--- a/pkgs/build-support/nix-gitignore/default.nix
+++ b/pkgs/build-support/nix-gitignore/default.nix
@@ -20,14 +20,13 @@ let
 in rec {
   # [["good/relative/source/file" true] ["bad.tmpfile" false]] -> root -> path
   filterPattern = patterns: root:
-    (name: _type:
-      let
-        relPath = lib.removePrefix ((toString root) + "/") name;
-        matches = pair: (match (head pair) relPath) != null;
-        matched = map (pair: [(matches pair) (last pair)]) patterns;
-      in
-        last (last ([[true true]] ++ (filter head matched)))
-    );
+    let
+      filters = map (pair: relPath: if match (head pair) relPath == null then true else last pair) patterns;
+    in
+      name: _type:
+        let
+          relPath = lib.removePrefix ((toString root) + "/") name;
+        in foldl' (acc: f: if acc == true then f relPath else acc) true filters;
 
   # string -> [[regex bool]]
   gitignoreToPatterns = gitignore:
@@ -91,7 +90,9 @@ in rec {
       (filter (l: !isList l && !isComment l)
       (split "\n" gitignore));
 
-  gitignoreFilter = ign: root: filterPattern (gitignoreToPatterns ign) root;
+  gitignoreFilter = ign: let
+    patterns = gitignoreToPatterns ign;
+  in root: filterPattern patterns root;
 
   # string|[string|file] (→ [string|file] → [string]) -> string
   gitignoreCompileIgnore = file_str_patterns: root:
@@ -100,9 +101,10 @@ in rec {
       str_patterns = map (onPath readFile) (lib.toList file_str_patterns);
     in concatStringsSep "\n" str_patterns;
 
-  gitignoreFilterPure = filter: patterns: root: name: type:
-    gitignoreFilter (gitignoreCompileIgnore patterns root) root name type
-    && filter name type;
+  gitignoreFilterPure = filter: patterns: root: let
+    compiledFilter = gitignoreCompileIgnore patterns root;
+    filterFn = gitignoreFilter compiledFilter;
+  in name: type: filterFn root name type && filter name type;
 
   # This is a very hacky way of programming this!
   # A better way would be to reuse existing filtering by making multiple gitignore functions per each root.