Speed up git diff highlight generation

go.mod

@@ -59,6 +59,7 @@ require (
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
 	github.com/hashicorp/go-retryablehttp v0.7.0 // indirect
 	github.com/hashicorp/go-version v1.3.1
+	github.com/hashicorp/golang-lru v0.5.1
 	github.com/huandu/xstrings v1.3.2
 	github.com/issue9/identicon v1.2.0
 	github.com/jaytaylor/html2text v0.0.0-20200412013138-3577fbdbcff7

go.sum

@@ -593,6 +593,7 @@ github.com/hashicorp/go-uuid v1.0.0/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/b
 github.com/hashicorp/go-uuid v1.0.1/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
 github.com/hashicorp/go.net v0.0.1/go.mod h1:hjKkEWcCURg++eb33jQU7oqQcI9XDCnUzHA0oac0k90=
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
+github.com/hashicorp/golang-lru v0.5.1 h1:0hERBMJE1eitiLkihrMvRVBYAkpHzc/J3QdDN+dAcgU=
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/hcl v1.0.0 h1:0Anlzjpi4vEasTeNFn2mLJgTSwt0+6sfsiTG8qcWGx4=
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=

modules/highlight/highlight.go

@@ -8,6 +8,7 @@ package highlight
 import (
 	"bufio"
 	"bytes"
+	"fmt"
 	gohtml "html"
 	"path/filepath"
 	"strings"
@@ -20,6 +21,7 @@ import (
 	"github.com/alecthomas/chroma/formatters/html"
 	"github.com/alecthomas/chroma/lexers"
 	"github.com/alecthomas/chroma/styles"
+	lru "github.com/hashicorp/golang-lru"
 )
 
 // don't index files larger than this many bytes for performance purposes
@@ -30,6 +32,8 @@ var (
 	highlightMapping = map[string]string{}
 
 	once sync.Once
+
+	cache *lru.ARCCache
 )
 
 // NewContext loads custom highlight map from local config
@@ -39,6 +43,13 @@ func NewContext() {
 		for i := range keys {
 			highlightMapping[keys[i].Name()] = keys[i].Value()
 		}
+
+		// The size 512 is simply a conservative rule of thumb
+		c, err := lru.NewARC(512)
+		if err != nil {
+			panic(fmt.Sprintf("failed to initialize LRU cache for highlighter: %s", err))
+		}
+		cache = c
 	})
 }
 
@@ -73,11 +84,18 @@ func Code(fileName, code string) string {
 		lexer = lexers.Get(val)
 	}
 
+	if lexer == nil {
+		if l, ok := cache.Get(fileName); ok {
+			lexer = l.(chroma.Lexer)
+		}
+	}
+
 	if lexer == nil {
 		lexer = lexers.Match(fileName)
 		if lexer == nil {
 			lexer = lexers.Fallback
 		}
+		cache.Add(fileName, lexer)
 	}
 
 	iterator, err := lexer.Tokenise(nil, string(code))