Merge 'Add support for substr scalar function' from Kim Seon Woo

Add support for `substr` scalar function. We can reuse the `substring` logic which is already implemted.

## Related issue
https://github.com/penberg/limbo/issues/144

Closes #289

COMPAT.md

@@ -104,8 +104,8 @@ This document describes the SQLite compatibility status of Limbo:
 | sqlite_offset(X)             | No     |         |
 | sqlite_source_id()           | No     |         |
 | sqlite_version()             | No     |         |
-| substr(X,Y,Z)                | No     |         |
-| substr(X,Y)                  | No     |         |
+| substr(X,Y,Z)                | Yes    |         |
+| substr(X,Y)                  | Yes    |         |
 | substring(X,Y,Z)             | Yes    |         |
 | substring(X,Y)               | Yes    |         |
 | total_changes()              | No     |         |

core/function.rs

@@ -54,6 +54,7 @@ pub enum ScalarFunc {
     Length,
     Min,
     Max,
+    Substr,
     Substring,
     Date,
     Time,
@@ -77,6 +78,7 @@ impl ToString for ScalarFunc {
             ScalarFunc::Length => "length".to_string(),
             ScalarFunc::Min => "min".to_string(),
             ScalarFunc::Max => "max".to_string(),
+            ScalarFunc::Substr => "substr".to_string(),
             ScalarFunc::Substring => "substring".to_string(),
             ScalarFunc::Date => "date".to_string(),
             ScalarFunc::Time => "time".to_string(),
@@ -117,6 +119,7 @@ impl Func {
             "rtrim" => Ok(Func::Scalar(ScalarFunc::RTrim)),
             "round" => Ok(Func::Scalar(ScalarFunc::Round)),
             "length" => Ok(Func::Scalar(ScalarFunc::Length)),
+            "substr" => Ok(Func::Scalar(ScalarFunc::Substr)),
             "substring" => Ok(Func::Scalar(ScalarFunc::Substring)),
             "date" => Ok(Func::Scalar(ScalarFunc::Date)),
             "time" => Ok(Func::Scalar(ScalarFunc::Time)),

core/translate/expr.rs

@@ -317,7 +317,7 @@ pub fn translate_expr(
                             });
                             Ok(target_register)
                         }
-                        ScalarFunc::Substring => {
+                        ScalarFunc::Substr | ScalarFunc::Substring => {
                             let args = if let Some(args) = args {
                                 if !(args.len() == 2 || args.len() == 3) {
                                     crate::bail_parse_error!(

core/vdbe/mod.rs

@@ -1313,7 +1313,7 @@ impl Program {
                         }
                         state.pc += 1;
                     }
-                    Func::Scalar(ScalarFunc::Substring) => {
+                    Func::Scalar(ScalarFunc::Substr) | Func::Scalar(ScalarFunc::Substring) => {
                         let start_reg = *start_reg;
                         let str_value = &state.registers[start_reg];
                         let start_value = &state.registers[start_reg + 1];

testing/scalar-functions.test

@@ -255,6 +255,22 @@ do_execsql_test max-null {
   select max(null,null)
 } {}
 
+do_execsql_test substr-3-args {
+  SELECT substr('limbo', 1, 3);
+} {lim}
+
+do_execsql_test substr-3-args-exceed-length {
+  SELECT substr('limbo', 1, 10);
+} {limbo}
+
+do_execsql_test substr-3-args-start-exceed-length {
+  SELECT substr('limbo', 10, 3);
+} {}
+
+do_execsql_test substr-2-args {
+  SELECT substr('limbo', 3);
+} {mbo}
+
 do_execsql_test substring-3-args {
   SELECT substring('limbo', 1, 3);
 } {lim}