fold: variable width tabs, guard treating tab as whitespace

Treat tab chars as advancing to the next tab stop rather than having a fixed
8-column width.

Also treat tab as a whitespace split target only when splitting on word
boundaries.

src/uu/fold/src/fold.rs

@@ -14,6 +14,8 @@ use std::fs::File;
 use std::io::{stdin, BufRead, BufReader, Read};
 use std::path::Path;
 
+const TAB_WIDTH: usize = 8;
+
 static SYNTAX: &str = "[OPTION]... [FILE]...";
 static SUMMARY: &str = "Writes each file (or standard input if no files are given)
  to standard output whilst breaking long lines";
@@ -220,11 +222,14 @@ fn fold_file<T: Read>(mut file: BufReader<T>, spaces: bool, width: usize) {
 
             match ch {
                 '\t' => {
-                    if col_count + 8 > width && !output.is_empty() {
+                    let next_tab_stop = col_count + TAB_WIDTH - col_count % TAB_WIDTH;
+
+                    if next_tab_stop > width && !output.is_empty() {
                         emit_output!();
                     }
-                    col_count += 8;
-                    last_space = Some(char_count);
+
+                    col_count = next_tab_stop;
+                    last_space = if spaces { Some(char_count) } else { None };
                 }
                 '\x08' => {
                     // FIXME: does not match GNU's handling of backspace

tests/by-util/test_fold.rs

@@ -132,7 +132,7 @@ fn test_single_tab_should_not_add_extra_newline() {
 }
 
 #[test]
-fn test_tab_counts_as_8_columns() {
+fn test_initial_tab_counts_as_8_columns() {
     new_ucmd!()
         .arg("-w8")
         .pipe_in("\t1")
@@ -140,6 +140,33 @@ fn test_tab_counts_as_8_columns() {
         .stdout_is("\t\n1");
 }
 
+#[test]
+fn test_tab_should_advance_to_next_tab_stop() {
+    // tab advances the column count to the next tab stop, i.e. the width
+    // of the tab varies based on the leading text
+    new_ucmd!()
+        .args(&["-w8", "tab_stops.input"])
+        .succeeds()
+        .stdout_is_fixture("tab_stops_w8.expected");
+}
+
+#[test]
+fn test_all_tabs_should_advance_to_next_tab_stops() {
+    new_ucmd!()
+        .args(&["-w16", "tab_stops.input"])
+        .succeeds()
+        .stdout_is_fixture("tab_stops_w16.expected");
+}
+
+#[test]
+fn test_fold_before_tab_with_narrow_width() {
+    new_ucmd!()
+        .arg("-w7")
+        .pipe_in("a\t1")
+        .succeeds()
+        .stdout_is("a\n\t\n1");
+}
+
 #[test]
 fn test_fold_at_word_boundary() {
     new_ucmd!()
@@ -167,8 +194,35 @@ fn test_fold_at_word_boundary_preserve_final_newline() {
         .stdout_is("one \ntwo\n");
 }
 
+#[test]
+fn test_fold_at_tab() {
+    new_ucmd!()
+        .arg("-w8")
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\t\nbbb\n");
+}
+
+#[test]
+fn test_fold_after_tab() {
+    new_ucmd!()
+        .arg("-w10")
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\tbb\nb\n");
+}
+
 #[test]
 fn test_fold_at_tab_as_word_boundary() {
+    new_ucmd!()
+        .args(&["-w8", "-s"])
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\t\nbbb\n");
+}
+
+#[test]
+fn test_fold_after_tab_as_word_boundary() {
     new_ucmd!()
         .args(&["-w10", "-s"])
         .pipe_in("a\tbbb\n")
@@ -317,6 +371,15 @@ fn test_tab_counts_as_one_byte() {
         .stdout_is("1\t\n2\n");
 }
 
+#[test]
+fn test_bytewise_fold_before_tab_with_narrow_width() {
+    new_ucmd!()
+        .args(&["-w7", "-b"])
+        .pipe_in("a\t1")
+        .succeeds()
+        .stdout_is("a\t1");
+}
+
 #[test]
 fn test_bytewise_fold_at_word_boundary_only_whitespace() {
     new_ucmd!()

tests/fixtures/fold/tab_stops.input

@@ -0,0 +1,11 @@
+1	
+12	
+123	
+1234	
+12345	
+123456	
+1234567	
+12345678	
+123456781	
+12345678	2
+12345678	2	4

tests/fixtures/fold/tab_stops_w16.expected

@@ -0,0 +1,13 @@
+1	
+12	
+123	
+1234	
+12345	
+123456	
+1234567	
+12345678	
+123456781	
+12345678	
+2
+12345678	
+2	4

tests/fixtures/fold/tab_stops_w8.expected

@@ -0,0 +1,18 @@
+1	
+12	
+123	
+1234	
+12345	
+123456	
+1234567	
+12345678
+	
+12345678
+1	
+12345678
+	
+2
+12345678
+	
+2	
+4