feat: add prompt placeholder substitution with {filename} syntax

tests/test_prompt.py

@@ -0,0 +1,109 @@
+"""Tests for hokusai.prompt — prompt resolution and placeholder substitution."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from hokusai.prompt import (
+    extract_placeholder_files,
+    resolve_prompt,
+    substitute_placeholders,
+)
+
+
+class TestSubstitutePlaceholders:
+    """Test placeholder substitution in prompt text."""
+
+    def test_no_placeholders(self, project_dir: Path) -> None:
+        assert substitute_placeholders("Hello world", project_dir) == "Hello world"
+
+    def test_single_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "name.txt").write_text("World")
+        result = substitute_placeholders("Hello {name.txt}", project_dir)
+        assert result == "Hello World"
+
+    def test_multiple_placeholders(self, project_dir: Path) -> None:
+        _ = (project_dir / "first.txt").write_text("Alice")
+        _ = (project_dir / "last.txt").write_text("Smith")
+        result = substitute_placeholders("Dear {first.txt} {last.txt}", project_dir)
+        assert result == "Dear Alice Smith"
+
+    def test_escaped_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "name.txt").write_text("World")
+        result = substitute_placeholders("Hello \\{name.txt}", project_dir)
+        assert result == "Hello {name.txt}"
+
+    def test_double_escaped_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "name.txt").write_text("World")
+        result = substitute_placeholders("Hello \\\\{name.txt}", project_dir)
+        assert result == "Hello \\World"
+
+    def test_triple_escaped_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "name.txt").write_text("World")
+        result = substitute_placeholders("Hello \\\\\\{name.txt}", project_dir)
+        assert result == "Hello \\{name.txt}"
+
+    def test_placeholder_at_start(self, project_dir: Path) -> None:
+        _ = (project_dir / "greeting.txt").write_text("Hi there")
+        result = substitute_placeholders("{greeting.txt}!", project_dir)
+        assert result == "Hi there!"
+
+    def test_placeholder_with_multiline_content(self, project_dir: Path) -> None:
+        _ = (project_dir / "block.txt").write_text("line1\nline2\nline3")
+        result = substitute_placeholders("Start: {block.txt} :End", project_dir)
+        assert result == "Start: line1\nline2\nline3 :End"
+
+
+class TestExtractPlaceholderFiles:
+    """Test extraction of placeholder filenames from prompts."""
+
+    def test_no_placeholders(self) -> None:
+        assert extract_placeholder_files("Hello world") == []
+
+    def test_single_placeholder(self) -> None:
+        assert extract_placeholder_files("Hello {name.txt}") == ["name.txt"]
+
+    def test_multiple_placeholders(self) -> None:
+        result = extract_placeholder_files("{a.txt} and {b.txt}")
+        assert result == ["a.txt", "b.txt"]
+
+    def test_escaped_placeholder_ignored(self) -> None:
+        assert extract_placeholder_files("\\{name.txt}") == []
+
+    def test_double_escaped_included(self) -> None:
+        assert extract_placeholder_files("\\\\{name.txt}") == ["name.txt"]
+
+    def test_triple_escaped_ignored(self) -> None:
+        assert extract_placeholder_files("\\\\\\{name.txt}") == []
+
+
+class TestResolvePrompt:
+    """Test full prompt resolution (file loading + placeholder substitution)."""
+
+    def test_inline_prompt_no_placeholders(self, project_dir: Path) -> None:
+        assert resolve_prompt("Just a string", project_dir) == "Just a string"
+
+    def test_file_prompt(self, project_dir: Path) -> None:
+        _ = (project_dir / "prompt.txt").write_text("From file")
+        result = resolve_prompt("prompt.txt", project_dir)
+        assert result == "From file"
+
+    def test_nonexistent_file_treated_as_inline(self, project_dir: Path) -> None:
+        result = resolve_prompt("no_such_file.txt", project_dir)
+        assert result == "no_such_file.txt"
+
+    def test_inline_with_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "style.txt").write_text("impressionist")
+        result = resolve_prompt("Paint in {style.txt} style", project_dir)
+        assert result == "Paint in impressionist style"
+
+    def test_multiline_prompt_with_placeholder(self, project_dir: Path) -> None:
+        _ = (project_dir / "detail.txt").write_text("vivid colours")
+        result = resolve_prompt("First line\nWith {detail.txt}", project_dir)
+        assert result == "First line\nWith vivid colours"
+
+    def test_file_prompt_no_placeholder_processing(self, project_dir: Path) -> None:
+        """When the prompt is a file path, the file is loaded verbatim."""
+        _ = (project_dir / "prompt.txt").write_text("Literal {not_a_file.txt}")
+        result = resolve_prompt("prompt.txt", project_dir)
+        assert result == "Literal {not_a_file.txt}"