Auto merge of #365 - froydnj:uts-data-slimming, r=SimonSapin

more idna data slimming

We can do a better job of packing the uts46 data:

* We can merge identically-mapped entries that don't have an associated string slice.  This saves ~10% space.
* We can make slices smaller and pack them into `Mapping` better, which saves 25% space on 64-bit platforms.  I think it might save half that on 32-bit platforms, but I didn't check.

Together these are good for ~42KB of space savings on a 64-bit platform.

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idna/src/make_uts46_mapping_table.py

@@ -51,7 +51,15 @@ def strtab_slice(s):
         return c
 
 def rust_slice(s):
-    return "(StringTableSlice { byte_start: %d, byte_len: %d })" % s
+    start = s[0]
+    length = s[1]
+    start_lo = start & 0xff
+    start_hi = start >> 8
+    assert length <= 255
+    assert start_hi <= 255
+    return "(StringTableSlice { byte_start_lo: %d, byte_start_hi: %d, byte_len: %d })" % (start_lo, start_hi, length)
+
+ranges = []
 
 for line in txt:
     # remove comments
@@ -66,12 +74,58 @@ def rust_slice(s):
     if not last:
         last = first
     mapping = fields[1].strip().replace('_', ' ').title().replace(' ', '')
+    unicode_str = None
     if len(fields) > 2:
         if fields[2].strip():
             unicode_str = u''.join(char(c) for c in fields[2].strip().split(' '))
-            mapping += rust_slice(strtab_slice(unicode_str))
         elif mapping == "Deviation":
-            mapping += rust_slice(strtab_slice(''))
+            unicode_str = u''
+    ranges.append((first, last, mapping, unicode_str))
+
+def mergeable_key(r):
+    mapping = r[2]
+    # These types have associated data, so we should not merge them.
+    if mapping in ('Mapped', 'Deviation', 'DisallowedStd3Mapped'):
+        return r
+    assert mapping in ('Valid', 'Ignored', 'Disallowed', 'DisallowedStd3Valid')
+    return mapping
+
+grouped_ranges = itertools.groupby(ranges, key=mergeable_key)
+
+optimized_ranges = []
+
+for (k, g) in grouped_ranges:
+    group = list(g)
+    if len(group) == 1:
+        optimized_ranges.append(group[0])
+        continue
+    # Assert that nothing in the group has an associated unicode string.
+    for g in group:
+        if len(g[3]) > 2:
+            assert not g[3][2].strip()
+    # Assert that consecutive members of the group don't leave gaps in
+    # the codepoint space.
+    a, b = itertools.tee(group)
+    next(b, None)
+    for (g1, g2) in itertools.izip(a, b):
+        last_char = int(g1[1], 16)
+        next_char = int(g2[0], 16)
+        if last_char + 1 == next_char:
+            continue
+        # There's a gap where surrogates would appear, but we don't have to
+        # worry about that gap, as surrogates never appear in Rust strings.
+        # Assert we're seeing the surrogate case here.
+        assert last_char == 0xd7ff
+        assert next_char == 0xe000
+    first = group[0][0]
+    last = group[-1][1]
+    mapping = group[0][2]
+    unicode_str = group[0][3]
+    optimized_ranges.append((first, last, mapping, unicode_str))
+
+for (first, last, mapping, unicode_str) in optimized_ranges:
+    if unicode_str is not None:
+        mapping += rust_slice(strtab_slice(unicode_str))
     print("    Range { from: '%s', to: '%s', mapping: %s }," % (escape_char(char(first)),
                                                                 escape_char(char(last)),
                                                                 mapping))

idna/src/uts46.rs

@@ -21,17 +21,22 @@ include!("uts46_mapping_table.rs");
 
 #[derive(Debug)]
 struct StringTableSlice {
-    byte_start: u16,
-    byte_len: u16,
+    // Store these as separate fields so the structure will have an
+    // alignment of 1 and thus pack better into the Mapping enum, below.
+    byte_start_lo: u8,
+    byte_start_hi: u8,
+    byte_len: u8,
 }
 
 fn decode_slice(slice: &StringTableSlice) -> &'static str {
-    let start = slice.byte_start as usize;
+    let lo = slice.byte_start_lo as usize;
+    let hi = slice.byte_start_hi as usize;
+    let start = (hi << 8) | lo;
     let len = slice.byte_len as usize;
     &STRING_TABLE[start..(start + len)]
 }
 
-#[repr(u16)]
+#[repr(u8)]
 #[derive(Debug)]
 enum Mapping {
     Valid,