formatter: split lines significantly longer than others into separate…

… groups

pkg/format/alg.go

@@ -0,0 +1,45 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format
+
+import (
+	"iter"
+	"math"
+)
+
+// Splits fields into sub-slices based on their length to isolate fields or
+// groups of fields that are significantly longer than others in the group.
+//
+// The algorithm itself and the constants used in this function are from gofmt:
+// https://github.com/golang/go/blob/go1.23.0/src/go/printer/nodes.go#L126
+func splitSegmentedFields(fields []segmentedField) iter.Seq[[]segmentedField] {
+	return func(yield func([]segmentedField) bool) {
+		const r = 2.5
+		const smallSize = 40
+		var count, lower, size int
+		var lnsum float64
+		for i := 0; i < len(fields); i++ {
+			f := fields[i]
+			prevSize := size
+			size = len(f.typeName) + len(f.fieldName)
+			if size > 0 && prevSize > 0 && count > 0 && (prevSize > smallSize || size > smallSize) {
+				mean := math.Exp(lnsum / float64(count))
+				ratio := float64(size) / mean
+				if r*ratio <= 1 || r <= ratio {
+					// split the group
+					yield(fields[lower:i])
+					lower = i
+					count = 0
+					lnsum = 0
+				}
+			}
+			if size > 0 {
+				count++
+				lnsum += math.Log(float64(size))
+			}
+		}
+		yield(fields[lower:])
+	}
+}