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Artifact 51accc209953a47a899272be7bff0a92e5c6787797ece961a0903a99ef846381:

// Package highlight is quarry's thin chroma wrapper (0002 §2 lists the
// Forgejo one as [COPY]; theirs is coupled to their settings and caches,
// so this is a smaller fresh write). Output uses CSS classes, never
// inline styles, so the two-theme story stays in one stylesheet.
package highlight

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"regexp"
	"strings"
	"sync"

	"github.com/alecthomas/chroma/v2"
	chromahtml "github.com/alecthomas/chroma/v2/formatters/html"
	"github.com/alecthomas/chroma/v2/lexers"
	"github.com/alecthomas/chroma/v2/styles"
)

var formatter = chromahtml.New(
	chromahtml.WithClasses(true),
	chromahtml.PreventSurroundingPre(false),
)

// MaxBytes is a SAFETY VALVE, not a routine limit: past it, content is
// served as plain text rather than tokenised.
//
// It was briefly set at 256KB, which turned ten of sqlite's src files —
// btree.c, select.c, vdbe.c, where.c, expr.c among them — permanently
// grey. That is trading the feature for the fix. The real cost is not
// file size but the DOM: chroma emits a span per token, so a 198KB
// build.c becomes 40,083 spans and 137ms of DOMContentLoaded, against
// 29ms for a plain 407KB btree.c. Highlighting AFTER load does not help
// either — the same spans cost the same parse, just later.
//
// The answer is to highlight only what is on screen (windowing, the way
// the diff pages already work), at which point no threshold is needed.
// Until that lands this stays high enough to catch only content no
// reader is really reading, and the per-blob cache below carries the
// cost of everything else.
const MaxBytes = 2 << 20

// TooBig reports whether Code will decline this content on size alone,
// so a caller can say why rather than silently dropping the colour.
func TooBig(code []byte) bool { return len(code) > MaxBytes }

// cache memoises highlighted output. A blob is immutable, so its
// rendering is too: the key is the content hash plus the lexer hints,
// and a hit skips tokenising entirely.
//
// Bounded by TOTAL BYTES rather than entry count, because entries are
// wildly uneven — one btree.c is worth thousands of small files, and a
// count-based bound would hold a gigabyte or nothing depending on what
// was browsed. On overflow the map is dropped wholesale: a generational
// sweep with no bookkeeping, which for a read-through cache of
// immutable content costs one recompute rather than correctness.
var cache = struct {
	sync.Mutex
	m     map[string]string
	bytes int
}{m: map[string]string{}}

// cacheBudget is how much highlighted HTML to retain. Roughly a dozen
// large source files, or thousands of ordinary ones.
const cacheBudget = 64 << 20

func cacheKey(code []byte, language, filename string) string {
	sum := sha256.Sum256(code)
	return hex.EncodeToString(sum[:16]) + "\x00" + language + "\x00" + filename
}

func cacheGet(key string) (string, bool) {
	cache.Lock()
	defer cache.Unlock()
	v, ok := cache.m[key]
	return v, ok
}

func cachePut(key, html string) {
	cache.Lock()
	defer cache.Unlock()
	if cache.bytes+len(html) > cacheBudget {
		cache.m = map[string]string{}
		cache.bytes = 0
	}
	cache.m[key] = html
	cache.bytes += len(html)
}

// Code highlights source for the given language hint and/or filename;
// either may be empty. ok is false when no lexer matched, or when the
// content is past MaxBytes — callers fall back to an escaped <pre>.
func Code(code []byte, language, filename string) (string, bool) {
	if TooBig(code) {
		return "", false
	}
	key := cacheKey(code, language, filename)
	if html, ok := cacheGet(key); ok {
		return html, html != ""
	}
	html, ok := code_(code, language, filename)
	// A miss is cached too, as the empty string: "no lexer matched" is
	// as expensive to discover as a match and just as stable.
	cachePut(key, html)
	return html, ok
}

func code_(code []byte, language, filename string) (string, bool) {
	var lexer chroma.Lexer
	if language != "" {
		lexer = lexers.Get(language)
	}
	if lexer == nil && filename != "" {
		lexer = lexers.Match(filename)
	}
	// Content analysis ONLY for a shebang. Chroma's Analyse is eager
	// enough to read an English README as SQL (is/and/or/in/where are
	// all keywords) and paint the prose red; a #! line is the one
	// content signal that names a language on purpose.
	if lexer == nil && bytes.HasPrefix(code, []byte("#!")) {
		lexer = lexers.Analyse(string(code))
	}
	if lexer == nil {
		return "", false
	}
	lexer = chroma.Coalesce(lexer)
	it, err := lexer.Tokenise(nil, string(code))
	if err != nil {
		return "", false
	}
	var buf bytes.Buffer
	if err := formatter.Format(&buf, styles.Get("github"), it); err != nil {
		return "", false
	}
	return buf.String(), true
}

// CSS returns the chroma class rules for both themes: light rules bare,
// dark rules inside a prefers-color-scheme block — matching the page
// layout's theming (0002 §4 item 5: both themes).
//
// A chroma style only emits rules for token classes it colors
// explicitly, so a class present in the light output but absent from
// the dark output would fall through to its light (near-black) color on
// the dark background — invisible text (ticket 030). Every such class
// gets an explicit dark override at the base text color.
func CSS() string {
	var light, dark strings.Builder
	formatter.WriteCSS(&light, styles.Get("github"))
	formatter.WriteCSS(&dark, styles.Get("github-dark"))

	darkCSS := dark.String()
	darkClasses := map[string]bool{}
	for _, m := range classRule.FindAllStringSubmatch(darkCSS, -1) {
		darkClasses[m[1]] = true
	}
	base := "#e6edf3"
	if m := baseColor.FindStringSubmatch(darkCSS); m != nil {
		base = m[1]
	}
	var missing strings.Builder
	for _, m := range classRule.FindAllStringSubmatch(light.String(), -1) {
		if !darkClasses[m[1]] {
			fmt.Fprintf(&missing, ".chroma .%s { color: %s }\n", m[1], base)
			darkClasses[m[1]] = true
		}
	}
	// Three theme states, matching the page: the media query is guarded
	// against an explicit light choice, and the same rules repeat under
	// an explicit dark one. Without both, a reader who picks light on a
	// dark-mode device gets dark code colours on a light page, and
	// picking dark on a light-mode device does nothing at all.
	darkRules := darkCSS + missing.String()
	return light.String() +
		"\n@media (prefers-color-scheme: dark) {\n" +
		prefixRules(darkRules, `:root:not([data-theme="light"]) `) + "\n}\n" +
		"\n" + prefixRules(darkRules, `:root[data-theme="dark"] `) + "\n"
}

var (
	classRule = regexp.MustCompile(`\.chroma \.([a-zA-Z0-9]+) `)
	baseColor = regexp.MustCompile(`\.chroma \{[^}]*?color: (#[0-9a-fA-F]{3,8})`)
)

// prefixRules qualifies every ".chroma" selector with a theme scope, so
// one set of dark declarations can be emitted twice — once under the
// media query and once under the explicit choice — without writing the
// theme out by hand. A plain prefix rather than CSS nesting: same
// result, and it asks nothing of the browser.
func prefixRules(css, scope string) string {
	return strings.ReplaceAll(css, ".chroma ", scope+".chroma ")
}