feat: begin work on error printing

build.zig

@@ -113,6 +113,8 @@ pub fn build(b: *std.Build) void {
     const files = [_][]const u8{
         "src/01_lexic/root.zig",
         "src/02_syntax/root.zig",
+        "src/02_syntax/root.zig",
+        "src/errors/root.zig",
     };
     for (files) |file| {
         const file_unit_test = b.addTest(.{

src/errors/root.zig

@@ -20,16 +20,109 @@ pub const ErrorData = struct {
         };
     }
 
-    pub fn print(self: *@This()) void {
-        std.debug.print("Error: {s}\n", .{self.reason});
+    /// Generates an error string. `alloc` is used to create the string,
+    /// the caller should call `free` on the returning slice.
+    pub fn get_error_str(self: *@This(), source_code: []const u8, alloc: std.mem.Allocator) ![]u8 {
+        const faulty_line = get_line(source_code, self.start_position);
+
+        const error_message = try std.fmt.allocPrint(alloc,
+            \\Error: {s}
+            \\{s}
+        , .{ self.reason, faulty_line });
+
+        return error_message;
     }
 
+    // TODO:
+    // - transform absolute position into line:column
+    // - Get previous, current and next line
+    // - Display message
+
     /// Does nothing at the moment
     pub fn deinit(self: *@This()) void {
         _ = self;
     }
 };
 
+fn get_line(input: []const u8, at: usize) []const u8 {
+    var line_start: usize = 0;
+    var line_end: usize = 0;
+    var current_pos: usize = 0;
+    const cap = input.len;
+
+    // search the start pos of the line
+    while (current_pos < cap and current_pos < at) : (current_pos += 1) {
+        if (input[current_pos] == '\n' and current_pos + 1 < cap) {
+            line_start = current_pos + 1;
+        }
+    }
+
+    // search the end pos of the line
+    while (current_pos < cap) : (current_pos += 1) {
+        // EOF is EOL
+        if (current_pos + 1 == cap) {
+            line_end = current_pos + 1;
+            break;
+        }
+        if (input[current_pos] == '\n') {
+            // dont count the newline as part of the... line
+            line_end = current_pos;
+            break;
+        }
+    }
+
+    return input[line_start..line_end];
+}
+
 test {
     std.testing.refAllDecls(@This());
 }
+
+test "should get a single line" {
+    const input = "print(hello)";
+    const at = 4;
+    const output = get_line(input, at);
+
+    try std.testing.expectEqualStrings("print(hello)", output);
+}
+
+test "should get line from 2 lines (1)" {
+    const input = "print(hello)\nprint(bye)";
+    const at = 4;
+    const output = get_line(input, at);
+
+    try std.testing.expectEqualStrings("print(hello)", output);
+}
+
+test "should get line from 2 lines (2)" {
+    const input = "print(hello)\nprint(bye)";
+    const at = 15;
+    const output = get_line(input, at);
+
+    try std.testing.expectEqualStrings("print(bye)", output);
+}
+
+test "should get line from 2 lines (3)" {
+    const input = "print(hello)\nprint(sure?)\nprint(bye!)";
+    const at = 15;
+    const output = get_line(input, at);
+
+    try std.testing.expectEqualStrings("print(sure?)", output);
+}
+
+test "should gen error message" {
+    // get_error_str
+    const source = "print(ehh)";
+    var err = ErrorData{
+        .reason = "Invalid identifier",
+        .start_position = 6,
+        .end_position = 9,
+    };
+    const out = try err.get_error_str(source, std.testing.allocator);
+    defer std.testing.allocator.free(out);
+
+    try std.testing.expectEqualStrings(
+        \\Error: Invalid identifier
+        \\print(ehh)
+    , out);
+}