cgo: refactor constant expressions

Put them in a separate file for separation of concerns (making them
testable) and add some tests.

cgo/const.go

@@ -0,0 +1,59 @@
+package cgo
+
+// This file implements a parser of a subset of the C language, just enough to
+// parse common #define statements to Go constant expressions.
+
+import (
+	"go/ast"
+	"go/token"
+	"strings"
+)
+
+// parseConst parses the given string as a C constant.
+func parseConst(pos token.Pos, value string) *ast.BasicLit {
+	for len(value) != 0 && value[0] == '(' && value[len(value)-1] == ')' {
+		value = strings.TrimSpace(value[1 : len(value)-1])
+	}
+	if len(value) == 0 {
+		// Pretend it doesn't exist at all.
+		return nil
+	}
+	// For information about integer literals:
+	// https://en.cppreference.com/w/cpp/language/integer_literal
+	if value[0] == '"' {
+		// string constant
+		return &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: value}
+	}
+	if value[0] == '\'' {
+		// char constant
+		return &ast.BasicLit{ValuePos: pos, Kind: token.CHAR, Value: value}
+	}
+	// assume it's a number (int or float)
+	value = strings.Replace(value, "'", "", -1) // remove ' chars
+	value = strings.TrimRight(value, "lu")      // remove llu suffixes etc.
+	// find the first non-number
+	nonnum := byte(0)
+	for i := 0; i < len(value); i++ {
+		if value[i] < '0' || value[i] > '9' {
+			nonnum = value[i]
+			break
+		}
+	}
+	// determine number type based on the first non-number
+	switch nonnum {
+	case 0:
+		// no non-number found, must be an integer
+		return &ast.BasicLit{ValuePos: pos, Kind: token.INT, Value: value}
+	case 'x', 'X':
+		// hex integer constant
+		// TODO: may also be a floating point number per C++17.
+		return &ast.BasicLit{ValuePos: pos, Kind: token.INT, Value: value}
+	case '.', 'e':
+		// float constant
+		value = strings.TrimRight(value, "fFlL")
+		return &ast.BasicLit{ValuePos: pos, Kind: token.FLOAT, Value: value}
+	default:
+		// unknown type, ignore
+	}
+	return nil
+}