Fixing CjkHeavenlyStem and CjkEarthlyBranch

Refs #3

This isn't a full fallback system, as really we should have a Range on
NominalSystem for that. This implementation hopefully can be compatible
with general nominal system fallback handling.

CHANGELOG.md

@@ -11,6 +11,23 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Breaking Changes
 
 - `HangeulInformal` has been renamed to `HanjaInformal`, its correct name.
+- `DigitCollection::len` and `DigitCollection::digit` no longer receive a
+  `digit_index` parameter.
+- `DigitCollection::Fallback` is a new associated type that allows a digit
+  collection to define a fallback digit set. `NoFallback` is a new type that can
+  be used when no fallback should be used.
+
+### Added
+
+- `DigitCollection::fixed` is a new function that indicates the collection
+  should only ever be used to produce a single digit.
+
+### Fixed
+
+- `CjkHeavenlyStem` and `CjkEarthlyBranch` now fall back to `CjkDecimal` in a
+  fashion that matches the cSS specification. Specifically once the symbols have
+  been exhausted, the entire nominal formatting is produced by the fallback
+  rather than only formatting the remaining digits with the fallback.
 
 ## v0.2.2 (2023-01-18)
 

examples/previews.rs

@@ -201,7 +201,7 @@ where
     }
 
     fn preview_values(&self) -> Vec<u32> {
-        let count = u32::try_from(self.0.len(0)).expect("too many digits");
+        let count = u32::try_from(self.0.len()).expect("too many digits");
 
         let mut values = Vec::new();
         let base = u32::from(!(self.0.has_zero_digit() || self.0.zero_based()));
@@ -220,7 +220,7 @@ where
         values.push(base + count + 2);
 
         // Show the hundreds transition
-        let tens_count = u32::try_from(self.0.len(1)).expect("too many digits");
+        let tens_count = u32::try_from(self.0.len()).expect("too many digits");
         values.push(base + tens_count * count - 1);
         values.push(base + tens_count * count);
         values.push(base + tens_count * count + 1);