Allow RHS to be arbitrarily long in a pager

If stdout is not a TTY, assume it's a pager and don't truncate the
RHS.

CHANGELOG.md

@@ -9,6 +9,9 @@ blank lines between them to make it clearer.
 Fixed an issue where screen width was not shared evenly by LHS and
 RHS.
 
+Side-by-side display will now use the full width of the screen when
+using a pager (i.e. if stdout is a not a TTY).
+
 ### Parsing
 
 Fixed handling of `->` in Rust.

Cargo.lock

@@ -89,6 +89,7 @@ checksum = "524cbf6897b527295dff137cec09ecf3a05f4fddffd7dfcd1585403449e74198"
 name = "difftastic"
 version = "0.7.0"
 dependencies = [
+ "atty",
  "clap",
  "colored",
  "diff",

Cargo.toml

@@ -21,6 +21,7 @@ rust-embed = "5.9.0"
 rustc-hash = "1.1.0"
 strsim = "0.10.0"
 lazy_static = "1.4.0"
+atty = "0.2.14"
 
 [dev-dependencies]
 pretty_assertions = "0.6.1"

src/lines.rs

@@ -433,6 +433,23 @@ pub fn enforce_length(s: &str, line_length: usize) -> String {
     result
 }
 
+/// Truncate any lines in `s` that are longer than `line_length`.
+pub fn enforce_max_length(s: &str, line_length: usize) -> String {
+    let mut result = String::with_capacity(s.len());
+    for line in s.lines() {
+        // TODO: use length in chars not bytes.
+        if line.len() > line_length {
+            // Truncate.
+            result.push_str(&line[0..line_length]);
+            result.push('\n');
+        } else {
+            result.push_str(&format!("{}\n", line));
+        }
+    }
+
+    result
+}
+
 pub trait MaxLine {
     fn max_line(&self) -> LineNumber;
 }

src/side_by_side.rs

@@ -1,7 +1,8 @@
+use atty::Stream;
 use std::cmp::{max, min};
 use std::collections::HashMap;
 
-use crate::lines::{enforce_length, format_line_num, LineGroup, LineNumber};
+use crate::lines::{enforce_length, enforce_max_length, format_line_num, LineGroup, LineNumber};
 use crate::style::apply_colors;
 use crate::syntax::{aligned_lines, MatchedPos};
 
@@ -193,14 +194,19 @@ pub fn display(
         groups,
         lhs_formatted_length,
         rhs_column_width,
-        terminal_width,
+        if atty::is(Stream::Stdout) {
+            terminal_width
+        } else {
+            // Assume a pager like less.
+            1000
+        },
     );
 
     let lhs_content_width = lhs_formatted_length - lhs_column_width;
     let rhs_content_width = rhs_formatted_length - rhs_column_width;
 
     let lhs_src = enforce_length(lhs_src, lhs_content_width);
-    let rhs_src = enforce_length(rhs_src, rhs_content_width);
+    let rhs_src = enforce_max_length(rhs_src, rhs_content_width);
     let lhs_colored = apply_colors(&lhs_src, true, lhs_positions);
     let rhs_colored = apply_colors(&rhs_src, false, rhs_positions);