got the reporter doing the right thing without colour

README.md

@@ -1,2 +1,2 @@
-# pretty-diff-reporter
-Pretty reporter for cmp.Diff with colourized output support
+# multilinediff
+Pretty multiline diff tool and library with a custom reporter for cmp.Diff.

cmd/experiment/main.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/RageCage64/multilinediff"
+)
+
+func main() {
+	a := `a
+b
+c
+diff`
+	b := `a
+b
+c
+f
+f
+f`
+
+	fmt.Println(multilinediff.MultilineDiff(a, b, "\n"))
+}

cmd/mld/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("not ready yet")
+}

go.mod

@@ -1,4 +1,4 @@
-module github.com/RageCage64/pretty-diff-reporter
+module github.com/RageCage64/multilinediff
 
 go 1.18
 

multilinediff.go

@@ -0,0 +1,22 @@
+package multilinediff
+
+import (
+	"strings"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+)
+
+func MultilineDiff(a, b, lineSep string) string {
+	reporter := Reporter{
+		LineSep: lineSep,
+	}
+	cmp.Diff(
+		a, b,
+		cmpopts.AcyclicTransformer("multiline", func(s string) []string {
+			return strings.Split(s, lineSep)
+		}),
+		cmp.Reporter(&reporter),
+	)
+	return reporter.String()
+}

reporter.go

@@ -1,4 +1,4 @@
-package prettyreporter
+package multilinediff
 
 import (
 	"fmt"
@@ -7,10 +7,50 @@ import (
 	"github.com/google/go-cmp/cmp"
 )
 
+type diffType int
+
+const (
+	diffTypeEqual diffType = iota
+	diffTypeChange
+	diffTypeAdd
+)
+
+type diffLine struct {
+	diff diffType
+	old  string
+	new  string
+}
+
+func (l diffLine) toLine(length int) string {
+	line := ""
+
+	switch l.diff {
+	case diffTypeChange:
+		line += "- "
+	case diffTypeAdd:
+		line += "+ "
+	default:
+		line += "  "
+	}
+
+	line += l.old
+
+	for i := 0; i < length-len(l.old); i++ {
+		line += " "
+	}
+
+	line += "  "
+
+	line += l.new
+
+	return line
+}
+
 type Reporter struct {
+	LineSep string
+
 	path      cmp.Path
-	old       []string
-	new       []string
+	lines     []diffLine
 	diffCount int
 }
 
@@ -19,19 +59,25 @@ func (r *Reporter) PushStep(ps cmp.PathStep) {
 }
 
 func (r *Reporter) Report(rs cmp.Result) {
+	line := diffLine{}
 	vOld, vNew := r.path.Last().Values()
 	if !rs.Equal() {
 		r.diffCount++
 		if vOld.IsValid() {
-			r.old = append(r.old, fmt.Sprintf("%+v", vOld))
+			line.diff = diffTypeChange
+			line.old = fmt.Sprintf("%+v", vOld)
 		}
 		if vNew.IsValid() {
-			r.new = append(r.new, fmt.Sprintf("%+v", vNew))
+			if line.diff == diffTypeEqual {
+				line.diff = diffTypeAdd
+			}
+			line.new = fmt.Sprintf("%+v", vNew)
 		}
 	} else {
-		r.old = append(r.old, "")
-		r.new = append(r.new, fmt.Sprintf("%+v", vOld))
+		line.old = fmt.Sprintf("%+v", vOld)
+		line.new = fmt.Sprintf("%+v", vOld)
 	}
+	r.lines = append(r.lines, line)
 }
 
 func (r *Reporter) PopStep() {
@@ -39,16 +85,17 @@ func (r *Reporter) PopStep() {
 }
 
 func (r *Reporter) String() string {
-
-	return strings.Join(r.lines, "\n")
-}
-
-func maxLen(strs []string) int {
-	max := 0
-	for _, s := range strs {
-		if len(s) > max {
-			max = len(s)
+	maxLen := 0
+	for _, l := range r.lines {
+		if len(l.old) > maxLen {
+			maxLen = len(l.old)
 		}
 	}
-	return max
+
+	diffLines := []string{}
+	for _, l := range r.lines {
+		diffLines = append(diffLines, l.toLine(maxLen))
+	}
+
+	return strings.Join(diffLines, r.LineSep)
 }