From 41a02531e1102c19b0177baaed5f7a5db11226ee Mon Sep 17 00:00:00 2001
From: Jun Mukai <mukai@google.com>
Date: Mon, 12 Mar 2018 17:15:03 -0700
Subject: [PATCH] Fix patterns of provide_context gets panicked

This PR fixes two cases where provide_context panics even though
the previous call of `is_boundary` returns a PreContext error.

The primary cause is `provide_context` assumes its state as
Regional or Emoji; otherwise it gets panicked with 'invalid state!'
However:
- if the chunk starts with the cursor position, it requires
  pre-context for `cat_before` (and that's necessary for some
  cases indeed). so provide_context should fill `cat_before`
  in such case.
- if both `cat_before` and `cat_after` are RIS and the boundary
  is undecided by the chunk, it requires pre-context but it does
  not set its state to Regional. This is done by setting state
  within `handle_regional`.
---
 src/grapheme.rs | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/grapheme.rs b/src/grapheme.rs
index c07fc8c..69b96b3 100644
--- a/src/grapheme.rs
+++ b/src/grapheme.rs
@@ -360,7 +360,10 @@ impl GraphemeCursor {
         match self.state {
             GraphemeState::Regional => self.handle_regional(chunk, chunk_start),
             GraphemeState::Emoji => self.handle_emoji(chunk, chunk_start),
-            _ => panic!("invalid state")
+            _ => if self.cat_before.is_none() && self.offset == chunk.len() + chunk_start {
+                let ch = chunk.chars().rev().next().unwrap();
+                self.cat_before = Some(gr::grapheme_category(ch));
+            },
         }
     }
 
@@ -406,6 +409,7 @@ impl GraphemeCursor {
             return;
         }
         self.pre_context_offset = Some(chunk_start);
+        self.state = GraphemeState::Regional;
     }
 
     fn handle_emoji(&mut self, chunk: &str, chunk_start: usize) {
@@ -428,6 +432,7 @@ impl GraphemeCursor {
             return;
         }
         self.pre_context_offset = Some(chunk_start);
+        self.state = GraphemeState::Emoji;
     }
 
     /// Determine whether the current cursor location is a grapheme cluster boundary.
@@ -659,3 +664,21 @@ impl GraphemeCursor {
         }
     }
 }
+
+#[test]
+fn test_grapheme_cursor_ris_precontext() {
+    let s = "\u{1f1fa}\u{1f1f8}\u{1f1fa}\u{1f1f8}\u{1f1fa}\u{1f1f8}";
+    let mut c = GraphemeCursor::new(8, s.len(), true);
+    assert_eq!(c.is_boundary(&s[4..], 4), Err(GraphemeIncomplete::PreContext(4)));
+    c.provide_context(&s[..4], 0);
+    assert_eq!(c.is_boundary(&s[4..], 4), Ok(true));
+}
+
+#[test]
+fn test_grapheme_cursor_chunk_start_require_precontext() {
+    let s = "\r\n";
+    let mut c = GraphemeCursor::new(1, s.len(), true);
+    assert_eq!(c.is_boundary(&s[1..], 1), Err(GraphemeIncomplete::PreContext(1)));
+    c.provide_context(&s[..1], 0);
+    assert_eq!(c.is_boundary(&s[1..], 1), Ok(false));
+}