treerack/trace.go

package treerack

import (
	"errors"
	"fmt"
	"io"
	"strings"
)

const (
	maxTraceLineWidth    = 108
	minEventSnippetWidth = 36
)

func traceLevelOffset(tr []TraceEntry) int {
	min := -1
	for _, tri := range tr {
		if min < 0 || tri.Level < min {
			min = tri.Level
		}
	}

	return min
}

func traceIndent(tr TraceEntry, levelOffset int) string {
	return strings.Join(make([]string, tr.Level-levelOffset+1), "  ")
}

func traceEventSymbol(tr TraceEntry) string {
	switch tr.Event {
	case Enter:
		return ">"
	case Success:
		return "<"
	case Fail:
		return "!"
	default:
		return "?"
	}
}

func traceEnterMessage(tr TraceEntry) string {
	return fmt.Sprintf("parsing %s at %d:%d", tr.Parser, tr.FromLine+1, tr.FromCol+1)
}

func traceSuccessMessage(tr TraceEntry) string {
	return fmt.Sprintf(
		"%s success from %d:%d to %d:%d",
		tr.Parser,
		tr.FromLine+1, tr.FromCol+1,
		tr.ToLine+1, tr.ToCol+1,
	)
}

func traceFailMessage(tr TraceEntry) string {
	if tr.Reason == "" {
		return fmt.Sprintf(
			"%s failed from %d:%d at %d:%d",
			tr.Parser,
			tr.FromLine+1, tr.FromCol+1,
			tr.ToLine+1, tr.ToCol+1,
		)
	}

	return fmt.Sprintf(
		"%s failed from %d:%d at %d:%d, %s",
		tr.Parser,
		tr.FromLine+1, tr.FromCol+1,
		tr.ToLine+1, tr.ToCol+1,
		tr.Reason,
	)
}

func traceEventMessage(tr TraceEntry) string {
	switch tr.Event {
	case Enter:
		return traceEnterMessage(tr)
	case Success:
		return traceSuccessMessage(tr)
	case Fail:
		return traceFailMessage(tr)
	default:
		return "?"
	}
}

func traceEventLine(tr TraceEntry, levelOffset int) string {
	i := traceIndent(tr, levelOffset)
	e := traceEventSymbol(tr)
	m := traceEventMessage(tr)
	return fmt.Sprintf("%s%s %s", i, e, m)
}

func traceEventSnippet(pe *ParseError, tr TraceEntry, maxLength int) string {
	if maxLength < minEventSnippetWidth {
		maxLength = minEventSnippetWidth
	}

	switch tr.Event {
	case Enter, Fail:
		if len(pe.InputContent(0, -1)) > tr.To {
			tr.To++
		}
	}

	minPos := tr.To - maxLength
	if minPos < 0 {
		minPos = 0
	}

	content := pe.InputContent(minPos, tr.To)
	lines := strings.Split(string(content), "\n")
	content = []rune(lines[len(lines)-1])
	if len(content) < maxLength && (len(lines) > 1 || minPos == 0) {
		return string(content)
	}

	if len(content)+3 > maxLength {
		content = content[len(content)+3-maxLength:]
	}

	return "..." + string(content)
}

// Trace checks if the provided error is of type *ParseError, and if yes, it writes the trace attached to the
// error to the provided output. If the err is not of type *ParseError or there is no trace attached, it does
// nothing and returns with nil. Note that it is possible and encouraged to generate custom visualizations of
// the parsing trace, from the structured trace entries attached to errors of type *ParseError, instead of using
// the Trace function.
func Trace(out io.Writer, err error) error {
	var perr *ParseError
	if !errors.As(err, &perr) {
		return nil
	}

	levelOffset := traceLevelOffset(perr.Trace)
	for _, tr := range perr.Trace {
		l := traceEventLine(tr, levelOffset)
		s := traceEventSnippet(perr, tr, maxTraceLineWidth-len(l)-4)
		if _, err := fmt.Fprintf(out, "%s: '%s'\n", l, s); err != nil {
			return err
		}
	}

	return nil
}