Support pi() function

COMPAT.md

@@ -184,7 +184,7 @@ Feature support of [sqlite expr syntax](https://www.sqlite.org/lang_expr.html).
 | log10(X)   | Yes    |         |
 | log2(X)    | Yes    |         |
 | mod(X,Y)   | Yes    |         |
-| pi()       | No     |         |
+| pi()       | Yes    |         |
 | pow(X,Y)   | Yes    |         |
 | power(X,Y) | Yes    |         |
 | radians(X) | Yes    |         |

core/translate/expr.rs

@@ -1604,6 +1604,20 @@ pub fn translate_expr(
                     }
                 }
                 Func::Math(math_func) => match math_func.arity() {
+                    MathFuncArity::Nullary => {
+                        if args.is_some() {
+                            crate::bail_parse_error!("{} function with arguments", math_func);
+                        }
+
+                        program.emit_insn(Insn::Function {
+                            constant_mask: 0,
+                            start_reg: 0,
+                            dest: target_register,
+                            func: func_ctx,
+                        });
+                        Ok(target_register)
+                    }
+
                     MathFuncArity::Unary => {
                         let args = if let Some(args) = args {
                             if args.len() != 1 {

core/vdbe/mod.rs

@@ -2492,6 +2492,10 @@ impl Program {
                             }
                         },
                         crate::function::Func::Math(math_func) => match math_func.arity() {
+                            MathFuncArity::Nullary => {
+                                state.registers[*dest] = OwnedValue::Float(std::f64::consts::PI);
+                            }
+
                             MathFuncArity::Unary => {
                                 let reg_value = &state.registers[*start_reg];
                                 let result = exec_math_unary(reg_value, math_func);
@@ -2504,6 +2508,7 @@ impl Program {
                                 let result = exec_math_binary(lhs, rhs, math_func);
                                 state.registers[*dest] = result;
                             }
+
                             _ => unimplemented!(),
                         },
                         crate::function::Func::Agg(_) => {

testing/math.test

@@ -364,6 +364,11 @@ do_execsql_test bitwise-not-zero {
 
 set tolerance 1e-13
 
+do_execsql_test_tolerance pi {
+  SELECT pi()
+} {3.14159265358979} $tolerance
+
+
 do_execsql_test_tolerance acos-int {
   SELECT acos(1)
 } {0.0} $tolerance