Wip errors reporting

errors.go

@@ -0,0 +1,147 @@
+package toml
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/pelletier/go-toml/v2/internal/unsafe"
+)
+
+// DecodeError represents an error encountered during the parsing or decoding
+// of a TOML document.
+//
+// In addition to the error message, it contains the position in the document
+// where it happened, as well as a human-readable representation that shows
+// where the error occurred in the document.
+type DecodeError struct {
+	message string
+	line    int
+	column  int
+
+	human string
+}
+
+// Error returns the error message contained in the DecodeError.
+func (e *DecodeError) Error() string {
+	return e.message
+}
+
+// String returns the human-readable contextualized error. This string is multi-line.
+func (e *DecodeError) String() string {
+	return e.human
+}
+
+/// Position returns the (line, column) pair indicating where the error
+// occurred in the document. Positions are 1-indexed.
+func (e *DecodeError) Position() (row int, column int) {
+	return e.line, e.column
+}
+
+// decodeErrorFromHighlight creates a DecodeError referencing to a highlighted
+// range of bytes from document.
+//
+// highlight needs to be a sub-slice of document, or this function panics.
+//
+// The function copies all bytes used in DecodeError, so that document and
+// highlight can be freely deallocated.
+func decodeErrorFromHighlight(document []byte, highlight []byte, message string) error {
+	err := &DecodeError{
+		message: message,
+	}
+
+	offset := unsafe.SubsliceOffset(document, highlight)
+
+	err.line, err.column = positionAtEnd(document[:offset])
+	before, after := linesOfContext(document, highlight, offset, 3)
+
+	var buf strings.Builder
+
+	maxLine := err.line + len(after) - 1
+	lineColumnWidth := len(strconv.Itoa(maxLine))
+
+	for i := len(before) - 1; i > 0; i-- {
+		line := err.line - i
+		buf.WriteString(formatLineNumber(line, lineColumnWidth))
+		buf.WriteString("| ")
+		buf.Write(before[i])
+		buf.WriteRune('\n')
+	}
+
+	buf.WriteString(formatLineNumber(err.line, lineColumnWidth))
+	buf.WriteString("| ")
+
+	if len(before) > 0 {
+		buf.Write(before[0])
+	}
+	buf.Write(highlight)
+	if len(after) > 0 {
+		buf.Write(after[0])
+	}
+	buf.WriteRune('\n')
+	buf.WriteString(strings.Repeat(" ", lineColumnWidth))
+	buf.WriteString("| ")
+	if len(before) > 0 {
+		buf.WriteString(strings.Repeat(" ", len(before[0])))
+	}
+	buf.WriteString(strings.Repeat("~", len(highlight)))
+	buf.WriteString(" ")
+	buf.WriteString(err.message)
+
+	for i := 1; i < len(after); i++ {
+		buf.WriteRune('\n')
+		line := err.line + i
+		buf.WriteString(formatLineNumber(line, lineColumnWidth))
+		buf.WriteString("| ")
+		buf.Write(after[i])
+	}
+
+	err.human = buf.String()
+	return err
+}
+
+func formatLineNumber(line int, width int) string {
+	format := "%" + strconv.Itoa(width) + "d"
+	return fmt.Sprintf(format, line)
+}
+
+func linesOfContext(document []byte, highlight []byte, offset int, linesAround int) ([][]byte, [][]byte) {
+	var beforeLines [][]byte
+	for beforeOffset, lastOffset := offset, offset; beforeOffset >= 0 && len(beforeLines) <= linesAround; beforeOffset-- {
+		if document[beforeOffset] == '\n' {
+			beforeLines = append(beforeLines, document[beforeOffset+1:lastOffset])
+			lastOffset = beforeOffset
+		} else if beforeOffset == 0 && beforeOffset != lastOffset {
+			beforeLines = append(beforeLines, document[beforeOffset:lastOffset])
+		}
+	}
+
+	var afterLines [][]byte
+
+	document = document[offset+len(highlight):]
+	for afterOffset, lastOffset := 0, 0; afterOffset < len(document) && len(afterLines) <= linesAround; afterOffset++ {
+		if document[afterOffset] == '\n' {
+			afterLines = append(afterLines, document[lastOffset:afterOffset])
+			afterOffset++ // skip \n
+			lastOffset = afterOffset
+		} else if afterOffset == len(document)-1 && lastOffset != afterOffset+1 {
+			afterLines = append(afterLines, document[lastOffset:afterOffset+1])
+		}
+	}
+	return beforeLines, afterLines
+}
+
+func positionAtEnd(b []byte) (row int, column int) {
+	row = 1
+	column = 1
+
+	for _, c := range b {
+		if c == '\n' {
+			row++
+			column = 1
+		} else {
+			column++
+		}
+	}
+	return
+}