Add Tabular Diff for CSV files (#14661)

Implements request #14320 The rendering of CSV files does match the diff style.

* Moved CSV logic into base package.

* Added method to create a tabular diff.

* Added CSV compare context.

* Added CSV diff template.

* Use new table style in CSV markup.

* Added file size limit for CSV rendering.

* Display CSV parser errors in diff.

* Lazy read single file.

* Lazy read rows for full diff.

* Added unit tests for various CSV changes.

modules/csv/csv.go

@@ -0,0 +1,93 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package csv
+
+import (
+	"bytes"
+	"encoding/csv"
+	"errors"
+	"regexp"
+	"strings"
+
+	"code.gitea.io/gitea/modules/translation"
+	"code.gitea.io/gitea/modules/util"
+)
+
+var quoteRegexp = regexp.MustCompile(`["'][\s\S]+?["']`)
+
+// CreateReader creates a csv.Reader with the given delimiter.
+func CreateReader(rawBytes []byte, delimiter rune) *csv.Reader {
+	rd := csv.NewReader(bytes.NewReader(rawBytes))
+	rd.Comma = delimiter
+	rd.TrimLeadingSpace = true
+	return rd
+}
+
+// CreateReaderAndGuessDelimiter tries to guess the field delimiter from the content and creates a csv.Reader.
+func CreateReaderAndGuessDelimiter(rawBytes []byte) *csv.Reader {
+	delimiter := guessDelimiter(rawBytes)
+	return CreateReader(rawBytes, delimiter)
+}
+
+// guessDelimiter scores the input CSV data against delimiters, and returns the best match.
+// Reads at most 10k bytes & 10 lines.
+func guessDelimiter(data []byte) rune {
+	maxLines := 10
+	maxBytes := util.Min(len(data), 1e4)
+	text := string(data[:maxBytes])
+	text = quoteRegexp.ReplaceAllLiteralString(text, "")
+	lines := strings.SplitN(text, "\n", maxLines+1)
+	lines = lines[:util.Min(maxLines, len(lines))]
+
+	delimiters := []rune{',', ';', '\t', '|', '@'}
+	bestDelim := delimiters[0]
+	bestScore := 0.0
+	for _, delim := range delimiters {
+		score := scoreDelimiter(lines, delim)
+		if score > bestScore {
+			bestScore = score
+			bestDelim = delim
+		}
+	}
+
+	return bestDelim
+}
+
+// scoreDelimiter uses a count & regularity metric to evaluate a delimiter against lines of CSV.
+func scoreDelimiter(lines []string, delim rune) float64 {
+	countTotal := 0
+	countLineMax := 0
+	linesNotEqual := 0
+
+	for _, line := range lines {
+		if len(line) == 0 {
+			continue
+		}
+
+		countLine := strings.Count(line, string(delim))
+		countTotal += countLine
+		if countLine != countLineMax {
+			if countLineMax != 0 {
+				linesNotEqual++
+			}
+			countLineMax = util.Max(countLine, countLineMax)
+		}
+	}
+
+	return float64(countTotal) * (1 - float64(linesNotEqual)/float64(len(lines)))
+}
+
+// FormatError converts csv errors into readable messages.
+func FormatError(err error, locale translation.Locale) (string, error) {
+	var perr *csv.ParseError
+	if errors.As(err, &perr) {
+		if perr.Err == csv.ErrFieldCount {
+			return locale.Tr("repo.error.csv.invalid_field_count", perr.Line), nil
+		}
+		return locale.Tr("repo.error.csv.unexpected", perr.Line, perr.Column), nil
+	}
+
+	return "", err
+}