sort: fix sorting by struct fields

vlib/builtin/array_test.v

@@ -736,6 +736,9 @@ fn test_sort() {
 	assert(users[0].age == 25)
 	assert(users[1].age == 22)
 	assert(users[2].age == 20)
+	//
+	users.sort(a.name < b.name) // Test sorting by string fields
+	//assert users.map(it.name).join(' ') == 'Alice Bob Peter'
 }
 
 fn test_f32_sort() {

vlib/cli/flag.v

@@ -183,3 +183,10 @@ fn (flags []Flag) contains(name string) bool {
 	return false
 }
 
+
+fn (mut flags []Flag) sort2() {
+       flags.sort_with_compare(fn (a, b &Flag) int {
+               return compare_strings(&a.name, &b.name)
+       })
+}
+

vlib/v/gen/cgen.v

@@ -24,10 +24,10 @@ const (
 )
 
 struct Gen {
-	table                 &table.Table
 	pref                  &pref.Preferences
 	module_built          string
 mut:
+	table                 &table.Table
 	out                   strings.Builder
 	cheaders              strings.Builder
 	includes              strings.Builder // all C #includes required by V modules
@@ -3739,14 +3739,14 @@ fn (mut g Gen) gen_array_map(node ast.CallExpr) {
 // `users.sort(a.age < b.age)`
 fn (mut g Gen) gen_array_sort(node ast.CallExpr) {
 	// println('filter s="$s"')
-	sym := g.table.get_type_symbol(node.return_type)
-	if sym.kind != .array {
+	return_sym := g.table.get_type_symbol(node.return_type)
+	if return_sym.kind != .array {
 		println(node.name)
 		println(g.typ(node.receiver_type))
-		println(sym.kind)
+		// println(sym.kind)
 		verror('sort() requires an array')
 	}
-	info := sym.info as table.Array
+	info := return_sym.info as table.Array
 	// No arguments means we are sorting an array of builtins (e.g. `numbers.sort()`)
 	// The type for the comparison fns is the type of the element itself.
 	mut typ := info.elem_type
@@ -3756,31 +3756,75 @@ fn (mut g Gen) gen_array_sort(node ast.CallExpr) {
 		// Get the type of the field that's being compared
 		// `a.age > b.age` => `age int` => int
 		infix_expr := node.args[0].expr as ast.InfixExpr
-		typ = infix_expr.left_type
+		// typ = infix_expr.left_type
 		is_reverse = infix_expr.op == .gt
 	}
-	mut compare_fn := match typ {
+	mut compare_fn := ''
+	match typ {
 		table.int_type {
-			'compare_ints'
+			compare_fn = 'compare_ints'
 		}
 		table.string_type {
-			'compare_strings'
+			compare_fn = 'compare_strings'
 		}
 		table.f64_type {
-			'compare_floats'
+			compare_fn = 'compare_floats'
 		}
 		else {
-			q := g.table.get_type_symbol(typ)
+			// Generate a comparison function for a custom type
 			if node.args.len == 0 {
-				verror('usage: .sort(a.field < b.field) $q.name')
-			}
-			verror('sort(): unhandled type $typ $q.name')
-			''
-		}
+				verror('usage: .sort(a.field < b.field)')
 			}
+			// verror('sort(): unhandled type $typ $q.name')
+			compare_fn = 'compare_' + g.typ(typ)
 			if is_reverse {
 				compare_fn += '_reverse'
 			}
+			if _ := g.table.find_fn(compare_fn) {
+			} else {
+				// Register a new custom `compare_xxx` function for qsort()
+				g.table.register_fn({
+					name: compare_fn
+					return_type: table.int_type
+				})
+				infix_expr := node.args[0].expr as ast.InfixExpr
+				styp := g.typ(typ)
+				// Variables `a` and `b` are used in the `.sort(a < b)` syntax, so we can reuse them
+				// when generating the function as long as the args are named the same.
+				g.definitions.writeln('int $compare_fn ($styp* a, $styp* b) {')
+				field_type := g.typ(infix_expr.left_type)
+				left_expr_str := g.write_expr_to_string(infix_expr.left).replace('.',
+					'->')
+				right_expr_str := g.write_expr_to_string(infix_expr.right).replace('.',
+					'->')
+				g.definitions.writeln('$field_type a_ = $left_expr_str;')
+				g.definitions.writeln('$field_type b_ = $right_expr_str;')
+				mut op1, mut op2 := '', ''
+				if infix_expr.left_type == table.string_type {
+					if is_reverse {
+						op1 = 'string_gt(a_, b_)'
+						op2 = 'string_lt(a_, b_)'
+					} else {
+						op1 = 'string_lt(a_, b_)'
+						op2 = 'string_gt(a_, b_)'
+					}
+				} else {
+					if is_reverse {
+						op1 = 'a_ > b_'
+						op2 = 'a_ < b_'
+					} else {
+						op1 = 'a_ < b_'
+						op2 = 'a_ > b_'
+					}
+				}
+				g.definitions.writeln('if ($op1) return -1;')
+				g.definitions.writeln('if ($op2) return 1; return 0; }\n')
+			}
+		}
+	}
+	if is_reverse && !compare_fn.ends_with('_reverse') {
+		compare_fn += '_reverse'
+	}
 	//
 	g.write('qsort(')
 	g.expr(node.left)