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

exists.go

@@ -1,31 +0,0 @@
-package util
-
-import "os"
-
-// FileExists reports whether there is a regular file at path. A directory, a
-// device node or a socket gives false, and so does anything the process is not
-// allowed to look at, since from the caller's point of view there is no usable
-// file there either way.
-//
-// Symbolic links are followed, so a link pointing at a regular file gives true
-// and a link pointing at nothing gives false.
-func FileExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-
-	return info.Mode().IsRegular()
-}
-
-// DirExists reports whether there is a directory at path. As with FileExists,
-// anything that cannot be looked at gives false, and symbolic links are
-// followed.
-func DirExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-
-	return info.IsDir()
-}

exists_test.go

@@ -1,57 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-func TestFileExistsAndDirExists(t *testing.T) {
-	root := t.TempDir()
-
-	file := filepath.Join(root, "a-file")
-	if err := os.WriteFile(file, []byte("contents"), 0644); err != nil {
-		t.Fatalf("could not write the test file: %v", err)
-	}
-
-	directory := filepath.Join(root, "a-directory")
-	if err := os.Mkdir(directory, 0755); err != nil {
-		t.Fatalf("could not make the test directory: %v", err)
-	}
-
-	linkToFile := filepath.Join(root, "link-to-file")
-	if err := os.Symlink(file, linkToFile); err != nil {
-		t.Fatalf("could not make the test link: %v", err)
-	}
-
-	brokenLink := filepath.Join(root, "broken-link")
-	if err := os.Symlink(filepath.Join(root, "nothing-here"), brokenLink); err != nil {
-		t.Fatalf("could not make the broken test link: %v", err)
-	}
-
-	tests := []struct {
-		name          string
-		path          string
-		expectedFile  bool
-		expectedIsDir bool
-	}{
-		{"a regular file", file, true, false},
-		{"a directory", directory, false, true},
-		{"a link to a file", linkToFile, true, false},
-		{"a link to nothing", brokenLink, false, false},
-		{"a path that is not there", filepath.Join(root, "missing"), false, false},
-		{"a path whose parent is not there", filepath.Join(root, "missing", "deeper"), false, false},
-		{"the empty path", "", false, false},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			if got := FileExists(test.path); got != test.expectedFile {
-				t.Errorf("FileExists: expected %v got %v", test.expectedFile, got)
-			}
-			if got := DirExists(test.path); got != test.expectedIsDir {
-				t.Errorf("DirExists: expected %v got %v", test.expectedIsDir, got)
-			}
-		})
-	}
-}

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)
+			}
+		})
+	}
+}