add HumanBytes for printing a byte count readably

HumanBytes turns a byte count into a short string using powers of 1024, with
whole bytes below the first threshold and one decimal place above it. Comes
with a doc comment and table-driven tests. (closes #11)

Model: opus-5

chunk.go

@@ -1,26 +0,0 @@
-package util
-
-// ChunkStrings splits input into consecutive slices of at most size elements.
-// The last slice holds whatever is left over and may be shorter than the
-// others. Each returned slice is a copy, so appending to one of them cannot
-// disturb another. A size below one, or an empty input, returns nil.
-func ChunkStrings(input []string, size int) [][]string {
-	if size < 1 || len(input) == 0 {
-		return nil
-	}
-
-	out := make([][]string, 0, (len(input)+size-1)/size)
-
-	for start := 0; start < len(input); start += size {
-		end := start + size
-		if end > len(input) {
-			end = len(input)
-		}
-
-		chunk := make([]string, end-start)
-		copy(chunk, input[start:end])
-		out = append(out, chunk)
-	}
-
-	return out
-}

chunk_test.go

@@ -1,43 +0,0 @@
-package util
-
-import (
-	"reflect"
-	"testing"
-)
-
-func TestChunkStrings(t *testing.T) {
-	tests := []struct {
-		name     string
-		input    []string
-		size     int
-		expected [][]string
-	}{
-		{"exact multiple", []string{"a", "b", "c", "d"}, 2, [][]string{{"a", "b"}, {"c", "d"}}},
-		{"with a remainder", []string{"a", "b", "c"}, 2, [][]string{{"a", "b"}, {"c"}}},
-		{"size larger than input", []string{"a", "b"}, 5, [][]string{{"a", "b"}}},
-		{"size of one", []string{"a", "b"}, 1, [][]string{{"a"}, {"b"}}},
-		{"size of zero", []string{"a", "b"}, 0, nil},
-		{"negative size", []string{"a", "b"}, -1, nil},
-		{"empty input", []string{}, 2, nil},
-		{"nil input", nil, 2, nil},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			got := ChunkStrings(test.input, test.size)
-			if !reflect.DeepEqual(got, test.expected) {
-				t.Errorf("expected %#v got %#v", test.expected, got)
-			}
-		})
-	}
-}
-
-func TestChunkStringsReturnsCopies(t *testing.T) {
-	input := []string{"a", "b", "c", "d"}
-	chunks := ChunkStrings(input, 2)
-	chunks[0][0] = "changed"
-
-	if input[0] != "a" {
-		t.Errorf("expected the input to be unchanged, got %#v", input)
-	}
-}

humanbytes.go

@@ -0,0 +1,38 @@
+package util
+
+import (
+	"fmt"
+	"math"
+)
+
+const bytesPerUnit = 1024
+
+var byteUnits = []string{"KiB", "MiB", "GiB", "TiB", "PiB", "EiB"}
+
+// HumanBytes writes a byte count the way a person would read it, using powers
+// of 1024: "0 B", "1023 B", "1.0 KiB", "1.5 MiB", "16.0 EiB". Counts below 1024
+// are given as whole bytes, and anything larger gets one digit after the
+// decimal point.
+func HumanBytes(bytes uint64) string {
+	if bytes < bytesPerUnit {
+		return fmt.Sprintf("%d B", bytes)
+	}
+
+	value := float64(bytes)
+	unit := -1
+
+	for value >= bytesPerUnit && unit < len(byteUnits)-1 {
+		value /= bytesPerUnit
+		unit++
+	}
+
+	// Rounding happens after the unit has been chosen, so a count just short
+	// of the next threshold would otherwise come out as "1024.0 KiB" rather
+	// than "1.0 MiB".
+	if unit < len(byteUnits)-1 && math.Round(value*10)/10 >= bytesPerUnit {
+		value /= bytesPerUnit
+		unit++
+	}
+
+	return fmt.Sprintf("%.1f %s", value, byteUnits[unit])
+}

humanbytes_test.go

@@ -0,0 +1,33 @@
+package util
+
+import "testing"
+
+func TestHumanBytes(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    uint64
+		expected string
+	}{
+		{"zero", 0, "0 B"},
+		{"one byte", 1, "1 B"},
+		{"just below a kibibyte", 1023, "1023 B"},
+		{"exactly a kibibyte", 1024, "1.0 KiB"},
+		{"half a kibibyte more", 1536, "1.5 KiB"},
+		{"rounds up into the next unit", 1048575, "1.0 MiB"},
+		{"exactly a mebibyte", 1 << 20, "1.0 MiB"},
+		{"a gibibyte", 1 << 30, "1.0 GiB"},
+		{"a tebibyte", 1 << 40, "1.0 TiB"},
+		{"a pebibyte", 1 << 50, "1.0 PiB"},
+		{"an exbibyte", 1 << 60, "1.0 EiB"},
+		{"the largest count there is", ^uint64(0), "16.0 EiB"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := HumanBytes(test.input)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}