normalize_stdout: maintain the elasticity of tabs

#117 (comment)

viddy currently breaks alignment for commands like `echo '1\t2\n\t3'` as
it always adds 4 spaces, this fixes that. One workaround for this is to
plumb the command via `expand` (`echo '1\t2\n\t3' | expand`).

Go version has the same issue, so this is not a regression.

src/bytes.rs

@@ -1,14 +1,18 @@
+const TAB_SIZE: usize = 4;
+
 pub fn normalize_stdout(b: Vec<u8>) -> Vec<u8> {
-    // To fix '\t' width issue replace '\t' with '    '
+    // Naively replace tabs ('\t') with at most `TAB_SIZE` spaces (' ') while
+    // maintaining the alignment / elasticity (see tests below).
     let mut b = b;
     let mut i = 0;
     while i < b.len() {
         if b[i] == b'\t' {
             b[i] = b' ';
-            b.insert(i, b' ');
-            b.insert(i, b' ');
-            b.insert(i, b' ');
-            i += 4;
+            let r = TAB_SIZE - (i % TAB_SIZE);
+            for _ in 1..r {
+                b.insert(i, b' ');
+            }
+            i += r - 1;
         } else {
             i += 1;
         }
@@ -21,8 +25,10 @@ mod test {
 
     #[test]
     fn test_normalize_stdout() {
-        let b = b"hello\tworld".to_vec();
-        let b = normalize_stdout(b);
-        assert_eq!(b, b"hello    world".to_vec());
+        assert_eq!(normalize_stdout(b"\t12345".to_vec()), b"    12345".to_vec());
+        assert_eq!(normalize_stdout(b"1\t2345".to_vec()), b"1   2345".to_vec());
+        assert_eq!(normalize_stdout(b"12\t345".to_vec()), b"12  345".to_vec());
+        assert_eq!(normalize_stdout(b"123\t45".to_vec()), b"123 45".to_vec());
+        assert_eq!(normalize_stdout(b"1234\t5".to_vec()), b"1234    5".to_vec());
     }
 }