html/render.go

package html

import (
	"bytes"
	"fmt"
	"io"
)

const defaultPWidth = 112

type renderGuide struct {
	inline   bool
	void     bool
	script   bool
	verbatim bool
}

type renderer struct {
	out           io.Writer
	indent        string
	pwidth        int
	currentIndent string
	err           error
}

func mergeRenderingGuides(rgs []renderGuide) renderGuide {
	var rg renderGuide
	for _, rgi := range rgs {
		rg.inline = rg.inline || rgi.inline
		rg.void = rg.void || rgi.void
		rg.script = rg.script || rgi.script
		rg.verbatim = rg.verbatim || rgi.verbatim
	}

	return rg
}

func attributeEscape(value string) string {
	var rr []rune
	r := []rune(value)
	for i := range r {
		switch r[i] {
		case '"':
			rr = append(rr, []rune(""")...)
		case '&':
			rr = append(rr, []rune("&")...)
		default:
			rr = append(rr, r[i])
		}
	}

	return string(rr)
}

func htmlEscape(s string) string {
	var (
		rr              []rune
		lastWS, wsStart bool
	)

	r := []rune(s)
	for i := range r {
		switch r[i] {
		case '<':
			rr = append(rr, []rune("&lt;")...)
		case '>':
			rr = append(rr, []rune("&gt;")...)
		case '&':
			rr = append(rr, []rune("&amp;")...)
		case ' ', 0xA0:
			if wsStart && lastWS {
				rr = append(rr[:len(rr)-1], []rune("&nbsp;&nbsp;")...)
			} else if lastWS {
				rr = append(rr, []rune("&nbsp;")...)
			} else {
				rr = append(rr, r[i])
			}
		default:
			rr = append(rr, r[i])
		}

		ws := r[i] == ' ' || r[i] == 0xA0
		wsStart = ws && !lastWS
		lastWS = ws
	}

	return string(rr)
}

func render(r *renderer, name string, children []any) {
	if r.err != nil {
		return
	}

	printf := func(f string, a ...any) {
		if r.err != nil {
			return
		}

		_, r.err = fmt.Fprintf(r.out, f, a...)
		if r.err != nil {
			r.err = fmt.Errorf("tag %s: %w", name, r.err)
		}
	}

	a, c, rgs := groupChildren(children)
	rg := mergeRenderingGuides(rgs)
	printf(r.currentIndent)
	printf("<%s", name)
	for _, ai := range a {
		for name, value := range ai {
			printf(" %s=\"%s\"", name, attributeEscape(value))
		}
	}

	printf(">")
	if r.indent != "" && !rg.inline && len(c) > 0 {
		printf("\n")
	}

	if rg.void {
		return
	}

	var inlineBuffer *bytes.Buffer
	if r.indent != "" {
		inlineBuffer = bytes.NewBuffer(nil)
	}

	// TODO:
	// - avoid rendering an inline buffer into another inline buffer
	// - why?
	// - or, if inline, just use the inline buffer without indentation
	// - check the wrapping again, if it preserves or eliminates the spaces the right way
	for i, ci := range c {
		if tag, ok := ci.(Tag); ok {
			if rg.inline {
				var rgq renderGuidesQuery
				tag(&rgq)
				crg := mergeRenderingGuides(rgq.value)
				if r.indent != "" && !crg.inline && inlineBuffer.Len() > 0 {
					w := r.pwidth
					if w == 0 {
						w = defaultPWidth
					}

					inlineBuffer = wrap(inlineBuffer, w, "")
					println(inlineBuffer.String())
					if _, err := io.Copy(r.out, inlineBuffer); err != nil {
						r.err = err
						return
					}

					inlineBuffer = bytes.NewBuffer(nil)
				}

				if i > 0 && r.indent != "" && !crg.inline {
					printf("\n")
				}

				rr := new(renderer)
				*rr = *r
				rr.indent = ""
				rr.currentIndent = ""
				tag(rr)
			} else {
				var rgq renderGuidesQuery
				tag(&rgq)
				crg := mergeRenderingGuides(rgq.value)
				if r.indent != "" && !crg.inline && inlineBuffer.Len() > 0 {
					w := r.pwidth
					if w == 0 {
						w = defaultPWidth
					}

					inlineBuffer = wrap(inlineBuffer, w, r.currentIndent+r.indent)
					println(inlineBuffer.String())
					if _, err := io.Copy(r.out, inlineBuffer); err != nil {
						r.err = err
						return
					}

					inlineBuffer = bytes.NewBuffer(nil)
				}

				if i > 0 && r.indent != "" && !crg.inline {
					printf("\n")
				}

				rr := new(renderer)
				*rr = *r
				rr.currentIndent += r.indent
				if r.indent != "" && crg.inline {
					rr.out = inlineBuffer
				}

				tag(rr)
			}

			continue
		}

		s := fmt.Sprint(ci)
		if s == "" {
			continue
		}

		if !rg.verbatim && !rg.script {
			s = htmlEscape(s)
		}

		if r.indent == "" {
			printf(s)
			continue
		}

		inlineBuffer.WriteString(s)
	}

	if r.indent != "" && inlineBuffer.Len() > 0 {
		w := r.pwidth
		if w == 0 {
			w = defaultPWidth
		}

		var indent string
		if !rg.inline && !rg.script {
			indent = r.currentIndent + r.indent
		}

		inlineBuffer = wrap(inlineBuffer, w, indent)
		if _, err := io.Copy(r.out, inlineBuffer); err != nil {
			r.err = err
			return
		}

		if !rg.inline {
			printf("\n")
		}
	}

	if !rg.inline {
		printf(r.currentIndent)
	}

	printf("</%s>", name)
	if r.indent != "" && !rg.inline {
		printf("\n")
	}
}