[TEM #2] - Add FileReader trait for testing

src/utils.rs

@@ -0,0 +1,50 @@
+use std::collections::HashMap;
+use std::fs;
+use std::path::{Path, PathBuf};
+
+use anyhow::{Context, Error, Result};
+
+pub(crate) trait FileReader {
+    fn read_to_string(&self, file_name: &Path, template_text: &str) -> Result<String>;
+}
+
+#[derive(PartialEq, Debug, Clone, Default)]
+pub(crate) struct SystemFileReader;
+
+#[derive(PartialEq, Debug, Clone, Default)]
+pub(crate) struct TestFileReader {
+    pub(crate) captured_contents: HashMap<PathBuf, String>,
+}
+
+impl FileReader for SystemFileReader {
+    fn read_to_string(&self, file_name: &Path, template_text: &str) -> Result<String> {
+        fs::read_to_string(file_name).with_context(|| {
+            format!(
+                "Could not read template file {} ({})",
+                template_text,
+                file_name.display(),
+            )
+        })
+    }
+}
+
+impl From<HashMap<PathBuf, String>> for TestFileReader {
+    fn from(map: HashMap<PathBuf, String>) -> Self {
+        TestFileReader {
+            captured_contents: map,
+        }
+    }
+}
+
+impl FileReader for TestFileReader {
+    fn read_to_string(&self, file_name: &Path, template_text: &str) -> Result<String> {
+        match self.captured_contents.get(file_name) {
+            Some(file_contents) => Ok(file_contents.to_string()),
+            None => Err(Error::msg(format!(
+                "Could not read template file {} ({})",
+                template_text,
+                file_name.display(),
+            ))),
+        }
+    }
+}