feat: added truncation_length/symbol to git_branch (#268)

Git branches can become very long (e.g. gitlab auto-generated branch
names), thus it would be nice to be able to truncate them to keep your
prompt lenght in line.

This patch adds two new options to the git_branch module:
* truncation_length: The amount of graphemes to of a gitbranch to
truncate to
* truncation_symbol: The symbol that should be used to indicate that a
branch name was trunctated

To be able to correctly work with UTF-8 graphemes, unicode-segmentation
was added as a dependency.

src/modules/git_branch.rs

@@ -1,4 +1,5 @@
 use ansi_term::Color;
+use unicode_segmentation::UnicodeSegmentation;
 
 use super::{Context, Module};
 
@@ -8,15 +9,54 @@ use super::{Context, Module};
 pub fn module<'a>(context: &'a Context) -> Option<Module<'a>> {
     const GIT_BRANCH_CHAR: &str = " ";
 
-    let branch_name = context.branch_name.as_ref()?;
     let segment_color = Color::Purple.bold();
 
     let mut module = context.new_module("git_branch")?;
     module.set_style(segment_color);
     module.get_prefix().set_value("on ");
 
+    let unsafe_truncation_length = module
+        .config_value_i64("truncation_length")
+        .unwrap_or(std::i64::MAX);
+    let truncation_symbol = get_graphemes(
+        module.config_value_str("truncation_symbol").unwrap_or("…"),
+        1,
+    );
+
     module.new_segment("symbol", GIT_BRANCH_CHAR);
-    module.new_segment("name", branch_name);
+
+    // TODO: Once error handling is implemented, warn the user if their config
+    // truncation length is nonsensical
+    let len = if unsafe_truncation_length <= 0 {
+        log::debug!(
+            "[WARN]: \"truncation_length\" should be a positive value, found {}",
+            unsafe_truncation_length
+        );
+        std::usize::MAX
+    } else {
+        unsafe_truncation_length as usize
+    };
+    let branch_name = context.branch_name.as_ref()?;
+    let truncated_graphemes = get_graphemes(&branch_name, len);
+    // The truncation symbol should only be added if we truncated
+    let truncated_and_symbol = if len < graphemes_len(&branch_name) {
+        truncated_graphemes + &truncation_symbol
+    } else {
+        truncated_graphemes
+    };
+
+    module.new_segment("name", &truncated_and_symbol);
 
     Some(module)
 }
+
+fn get_graphemes(text: &str, length: usize) -> String {
+    UnicodeSegmentation::graphemes(text, true)
+        .take(length)
+        .collect::<Vec<&str>>()
+        .concat()
+}
+
+fn graphemes_len(text: &str) -> usize {
+    UnicodeSegmentation::graphemes(&text[..], true).count()
+}