diff --git a/Tests/p4rseTests/SourceCodeTests/SourceCode.swift b/Tests/p4rseTests/SourceCodeTests/SourceCode.swift
new file mode 100644
index 0000000..925c56b
--- /dev/null
+++ b/Tests/p4rseTests/SourceCodeTests/SourceCode.swift
@@ -0,0 +1,252 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Foundation
+import Macros
+import P4Compiler
+import SwiftTreeSitter
+import SystemPackage
+import Testing
+import TreeSitter
+import TreeSitterP4
+
+@testable import Common
+
+@Test func test_preprocessor() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple.p4")
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+  #expect(#RequireOkResult(SpecialCompilers.ProgramCompiler.Compile(source.getSource())))
+  #expect(source.getLocations().getPath() == file)
+}
+
+@Test func test_preprocessor_search_for_file() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple.p4")
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+  #expect(#RequireOkResult(SpecialCompilers.ProgramCompiler.Compile(source.getSource())))
+  #expect(source.getLocations().getPath() == file)
+}
+
+@Test func test_preprocessor_nested_includes() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple-split.p4")
+
+  #expect(#RequireOkResult(prep.preprocess(file)))
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+  #expect(#RequireOkResult(SpecialCompilers.ProgramCompiler.Compile(source.getSource())))
+  #expect(source.getLocations().getPath() == file)
+}
+
+@Test func test_preprocessor_oneline_includes() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple-split-oneline.p4")
+
+  #expect(#RequireOkResult(prep.preprocess(file)))
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+  #expect(#RequireOkResult(SpecialCompilers.ProgramCompiler.Compile(source.getSource())))
+  #expect(source.getLocations().getPath() == file)
+}
+
+@Test func test_preprocessor_missing_file() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "./TestData/Sources/unfound.p4")
+
+  #expect(
+    #RequireErrorResult(
+      Error(withMessage: "Could not open ./TestData/Sources/unfound.p4 for preprocessing"),
+      (prep.preprocess(file))))
+
+}
+
+@Test func test_preprocessor_missing_included_file() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple-unfound.p4")
+
+  #expect(
+    #RequireErrorResult(
+      Error(withMessage: "Could not open unfound.p4 for preprocessing"), (prep.preprocess(file))))
+
+}
+
+@Test func test_preprocessor_no_change_locations() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "testing.p4")
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+
+  #expect(source.getLocations().getLocation() == SourceLocation(0..<173))
+}
+
+@Test func test_preprocessor_change_locations() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "testing-split-a.p4")
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+
+  #expect(source.getLocations().getLocation() == SourceLocation(0..<173))
+  #expect(source.getLocations().getNestedLocations()[0].getLocation() == SourceLocation(48..<173))
+}
+
+@Test func test_preprocessor_oneline_includes_locations() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "simple-split-oneline.p4")
+
+  #expect(#RequireOkResult(prep.preprocess(file)))
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+
+  #expect(source.getLocations().getLocation() == SourceLocation(0..<173))
+  #expect(source.getLocations().getNestedLocations()[0].getLocation() == SourceLocation(0..<70))
+  #expect(source.getLocations().getNestedLocations()[1].getLocation() == SourceLocation(70..<173))
+}
+
+@Test func test_preprocessor_nested_includes_locations() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "nested-split.p4")
+
+  #expect(#RequireOkResult(prep.preprocess(file)))
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+
+  #expect(source.getLocations().getLocation() == SourceLocation(0..<173))
+  /*
+  #expect(
+    source.getLocations().getNestedLocations()[0].getNestedLocations()[0].getLocation()
+      == SourceLocation(27..<47))
+  #expect(
+    source.getLocations().getNestedLocations()[0].getNestedLocations()[1].getLocation()
+      == SourceLocation(48..<166))
+      */
+}
+
+@Test func test_preprocessor_nested_includes_annotated_source() async throws {
+  let sm = SourceManager(["./TestData/Sources/"])
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "annotate.p4")
+
+  let expected = """
+    <struct Testing {
+    <    bool yesno;
+        int count;>
+    };
+    <parser main_parser() {
+    <    state start {
+            Testing ts;
+            ts.yesno = true;
+            ts.count = 5;
+            transition select (ts.count == 5) {
+                true: accept;
+                false: reject;
+            };
+        }
+    >
+    }>>
+    """
+
+  #expect(#RequireOkResult(prep.preprocess(file)))
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+  #expect(source.getSource(annotated: true) == expected)
+}
+
+@Test func test_preprocessor_nested_includes_get_file_location() async throws {
+  let sm = SourceManager(["./TestData/Sources/"], FileManager())  // Add a FileManager to get absolute paths.
+  let prep = SourceCodePreprocessor(sm)
+  let file = FilePath.init(stringLiteral: "file-loc.p4")
+
+  let source = try! (#UseOkResult(prep.preprocess(file)))
+
+  let expected_file = file
+  let expected_nested_file = FilePath(stringLiteral: "file-loc-parser.p4")
+  let expected_nested_nested_file = FilePath(stringLiteral: "file-loc-parser-state.p4")
+
+  let found_nested_files = try! #require(source.pathForLocation(78))
+
+  #expect(found_nested_files[0].path == expected_file)
+  #expect(found_nested_files[1].path == expected_nested_file)
+  #expect(found_nested_files[2].path == expected_nested_nested_file)
+}
+
+@Test func test_source_location_contains() async throws {
+  let outer = SourceLocation(0..<500)
+  let inner = SourceLocation(0..<499)
+
+  #expect(outer.contains(inner))
+}
+
+@Test func test_source_location_contains_a() async throws {
+  let outer = SourceLocation(0, 500)
+  let inner = SourceLocation(0, 499)
+
+  #expect(outer.contains(inner))
+}
+
+@Test func test_source_location_contains2() async throws {
+  let outer = SourceLocation(0..<500)
+  let not_inner = SourceLocation(0..<501)
+
+  #expect(!outer.contains(not_inner))
+}
+
+@Test func test_source_location_contains2_a() async throws {
+  let outer = SourceLocation(0, 500)
+  let not_inner = SourceLocation(0, 501)
+
+  #expect(!outer.contains(not_inner))
+}
+
+@Test func test_source_location_contains3() async throws {
+  let outer = SourceLocation(200..<500)
+  let inner = SourceLocation(200..<499)
+
+  #expect(outer.contains(inner))
+}
+
+@Test func test_source_location_contains3_a() async throws {
+  let outer = SourceLocation(200, 300)
+  let inner = SourceLocation(200, 299)
+
+  #expect(outer.contains(inner))
+}
+
+@Test func test_source_location_contains4() async throws {
+  let outer = SourceLocation(200..<500)
+  let inner = SourceLocation(200, 299)
+
+  #expect(outer.contains(inner))
+}
+
+@Test func test_source_location_contains5() async throws {
+  let outer = SourceLocation(200..<300)
+  let not_inner = SourceLocation(200, 101)
+
+  #expect(!outer.contains(not_inner))
+}