documentation: Document SourceCode-related structs

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/Common/SourceCode.swift

@@ -40,14 +40,49 @@ public struct SourceLocation: Equatable, CustomStringConvertible {
   }
 }
 
-/// Represent a set of directories containing P4 code that can be accessed with relative paths.
+/// Represent search paths for P4 code that can be accessed with relative paths.
 public struct SourceManager {
   let paths: [FilePath]
-  public init(_ paths: [FilePath]) {
-    self.paths = paths
+
+  /// Create a `SourceManager`
+  ///
+  /// Any relative `FilePath`s in `paths` will be absolutized
+  /// if a `FileManager` is given.
+  ///
+  /// parameters:
+  /// - paths: The include paths searched for files with relative paths.
+  /// - fm: An optional instance of a `FileManager` that will be used to
+  /// convert relative paths in `paths` to absolute paths.
+  public init(_ paths: [FilePath], _ fm: FileManager? = .none) {
+
+    // If the user gives a file manager, we will convert relative paths
+    // to absolute paths. Otherwise, we do not.
+    guard let fm else {
+      self.paths = paths
+      return
+    }
+
+    // There is a file manager, so we should try to absolutize any
+    // relative paths
+    self.paths = paths.map {
+      if !$0.isAbsolute {
+        return FilePath(fm.currentDirectoryPath + "/" + $0.string).lexicallyNormalized()
+      }
+      return $0
+    }
   }
 
+  /// Return `FilePath` of `file` in search paths.
+  ///
+  /// Only if `file` is relative will the search paths be searched.
+  ///
+  /// parameters:
+  /// - file: A file to look for in the search paths.
   public func firstExisting(_ file: FilePath) -> FilePath? {
+    if file.isAbsolute {
+      return file
+    }
+
     let fm = FileManager()
     for path in self.paths {
       let combined = path.pushing(file)
@@ -59,7 +94,10 @@ public struct SourceManager {
   }
 }
 
-/// Represent preprocessed P4 code and retain information about source filenames.
+/// Represent preprocessed P4 code
+///
+/// The preprocessed code has metadata to recover the paths of any
+/// code generated by a preprocessor directive.
 public struct FileSourceLocation: Equatable, CustomStringConvertible {
   let location: SourceLocation
   let path: FilePath