fix: add reference comments, test U+0131, etc.

to code & documentation

- add references for EU character mappings
  - mobilexag/cordova-sqlite-evplus-ext-free#21
  - storesafe/cordova-sqlite-evcore-extbuild-free#50 (comment)
- add reference to cordova-sqlite-storage test suite from:
  - https://github.com/storesafe/cordova-sqlite-storage/tree/6.0.0/spec
- add reference to:
  - https://www.compart.com/en/unicode/U+0131
- test one-way U+0131 (`ı`) -> capital I
- update reproduction & documentation of known issue with
  U+0131 (`ı`) & capital I

README.md

@@ -42,6 +42,18 @@ int main() {
   - `cc test.c sqlite3.c sqlite3_eu.c -o test`
   - `./test`
 
-## Known quirks
+## Known mapping issue
+
+`SELECT LOWER_EU('I')` gives a result with U+0131 (`ı`)
+
+Here is a manifestation, based on the cordova-sqlite-storage test suite:
 
 - Statements such as `SELECT LOWER_EU(9e999)` return results like `ınf`
+
+## References
+
+- https://github.com/mobilexag/cordova-sqlite-evplus-ext-free/issues/21
+- https://github.com/storesafe/cordova-sqlite-evcore-extbuild-free/issues/50#issuecomment-498858058
+- https://www.compart.com/en/unicode/U+0131
+- cordova-sqlite-storage test suite ref:
+  - https://github.com/storesafe/cordova-sqlite-storage/tree/6.0.0/spec

sqlite3_eu.c

@@ -40,6 +40,9 @@ void init_map() {
     }
   }
 
+  // references:
+  // - https://github.com/mobilexag/cordova-sqlite-evplus-ext-free/issues/21
+  // - https://github.com/storesafe/cordova-sqlite-evcore-extbuild-free/issues/50#issuecomment-498858058
   EU_MAP_ENTRY(0x00E4, 0x00C4);
   EU_MAP_ENTRY(0x00E0, 0x00C0);
   EU_MAP_ENTRY(0x00E1, 0x00C1);
@@ -74,6 +77,13 @@ void init_map() {
   EU_MAP_ENTRY(0x00ED, 0x00CD);
   EU_MAP_ENTRY(0x00EE, 0x00CE);
   EU_MAP_ENTRY(0x00EF, 0x00CF);
+  // XXX TODO: NEED ONE-WAY MAPPING FOR U+0131 -> U+0049
+  // TO AVOID KNOWN ISSUE WITH SELECT LOWER_EU(9e999)
+  // OR SELECT LOWER_EU(-9e999)
+  // see cordova-sqlite-storage test suite ref:
+  // - https://github.com/storesafe/cordova-sqlite-storage/tree/6.0.0/spec
+  // see also:
+  // - https://www.compart.com/en/unicode/U+0131
   EU_MAP_ENTRY(0x0131, 0x0049);
   EU_MAP_ENTRY(0x012B, 0x012A);
   EU_MAP_ENTRY(0x012F, 0x012E);

test.c

@@ -58,11 +58,25 @@ int main() {
   sqlite3_exec(db, "SELECT UPPER_EU('đ ď ð abc')",
     assert_result_matches, "Đ Ď Ð ABC", NULL);
 
-  // TBD STRANGE RESULT for this operation:
+  // ref:
+  // - https://www.compart.com/en/unicode/U+0131
+  // - https://github.com/mobilexag/cordova-sqlite-evplus-ext-free/issues/21
+  TESTLOG("TRY SELECT UPPER_EU('ı') - should be one-way mapping");
+  sqlite3_exec(db, "SELECT UPPER_EU('ı')", assert_result_matches, "I", NULL);
+
+  // XXX KNOWN ISSUE REPRODUCED AT THIS POINT:
+  TESTLOG("TRY SELECT LOWER_EU('I') - KNOWN ISSUE AT THIS POINT");
+  sqlite3_exec(db, "SELECT LOWER_EU('I')", assert_result_matches, "\u0131", NULL);
+
+  // XXX KNOWN ISSUE REPRODUCED AT THIS POINT:
+  // TBD STRANGE RESULT for this operation,
+  // see cordova-sqlite-storage test suite ref:
+  // - https://github.com/storesafe/cordova-sqlite-storage/tree/6.0.0/spec
   TESTLOG("SELECT LOWER_EU(9e999)");
   sqlite3_exec(db, "SELECT LOWER_EU(9e999)",
     assert_result_matches, "ınf", NULL);
 
+  // XXX KNOWN ISSUE REPRODUCED AT THIS POINT:
   // TBD STRANGE RESULT for this operation:
   TESTLOG("SELECT HEX(LOWER_EU(9e999))");
   sqlite3_exec(db, "SELECT HEX(LOWER_EU(9e999))",