package modules

import (
	"html/template"
	"strings"

	"github.com/alecthomas/chroma/v2"
	"github.com/alecthomas/chroma/v2/lexers"
)

// maxHighlightSize bounds how much source chroma will tokenise. Past this,
// a file falls back to plain (still viewable, just uncoloured) text rather
// than risk a slow lex on something pathological.
const maxHighlightSize = 512 * 1024

// highlightLines lexes src by the language implied by path and returns one
// escaped, span-wrapped HTML fragment per line. ok is false when path has no
// recognised language or src is too large to bother lexing; callers should
// fall back to plain escaped lines in that case.
func highlightLines(path string, src []byte) (lines []template.HTML, ok bool) {
	if len(src) > maxHighlightSize {
		return nil, false
	}
	lexer := lexers.Match(path)
	if lexer == nil {
		return nil, false
	}
	it, err := chroma.Coalesce(lexer).Tokenise(nil, string(src))
	if err != nil {
		return nil, false
	}
	for _, toks := range chroma.SplitTokensIntoLines(it.Tokens()) {
		var b strings.Builder
		for i, tok := range toks {
			// SplitTokensIntoLines keeps each line's newline attached to
			// its last token; drop it so it doesn't render as a blank
			// row inside the (white-space: pre) table cell.
			if i == len(toks)-1 {
				tok.Value = strings.TrimSuffix(tok.Value, "\n")
			}
			esc := template.HTMLEscapeString(tok.Value)
			if cls := tokenClass(tok.Type); cls != "" {
				b.WriteString(`<span class="`)
				b.WriteString(cls)
				b.WriteString(`">`)
				b.WriteString(esc)
				b.WriteString(`</span>`)
			} else {
				b.WriteString(esc)
			}
		}
		lines = append(lines, template.HTML(b.String()))
	}
	return lines, true
}

// tokenClass buckets chroma's ~70 token types down to the handful of colours
// the theme actually defines (see --syn-* in base.css). Punctuation,
// operators and plain names are left uncoloured on purpose -- a source line
// with every token tinted reads as noise, not signal.
func tokenClass(t chroma.TokenType) string {
	switch {
	case t.InCategory(chroma.Comment):
		return "tok-c"
	case t.InCategory(chroma.Keyword):
		return "tok-k"
	case t.InSubCategory(chroma.LiteralString):
		return "tok-s"
	case t.InSubCategory(chroma.LiteralNumber):
		return "tok-n"
	case t == chroma.NameFunction, t == chroma.NameFunctionMagic,
		t == chroma.NameClass, t == chroma.NameNamespace,
		t == chroma.NameBuiltin, t == chroma.NameBuiltinPseudo,
		t == chroma.NameTag, t == chroma.NameDecorator:
		return "tok-fn"
	default:
		return ""
	}
}