db-subclass-to-dto/tools/ExtractMethod/Tooling/RefactoringGenerator.cs

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace ExtractMethod.Tooling;

/// <summary>
/// The generated extract-method refactoring (yak em 04 codegen): the NEW
/// method (the em 03 promoted signature; parameter names come straight from
/// the source symbols), the CALL STATEMENTS that replace the selection in the
/// enclosing method, and the fully transformed syntax tree.
/// </summary>
/// <param name="NewMethod">The extracted method, ready to be read. Built
/// with SyntaxFactory and NORMALIZED (whitespace) before insertion, so its
/// text is the readable shape that lands in the file — not the trivia-less
/// builder output.</param>
/// <param name="CallStatements">The statement(s) that replace the selected
/// run at the call site (one statement, or a capture + destructure pair for
/// a mixed tuple return), normalized like <see cref="NewMethod"/>.</param>
/// <param name="TransformedTree">The whole input tree with the selection
/// moved out and the new method added next to the enclosing one — the
/// round-trip check compiles exactly this tree.</param>
public sealed record Refactoring(
    MethodDeclarationSyntax NewMethod,
    IReadOnlyList<StatementSyntax> CallStatements,
    SyntaxTree TransformedTree)
{
    public string TransformedText => TransformedTree.ToString();
}

/// <summary>
/// Emits the refactoring behind the em 03 suggestion.
///
/// WHY no identifier rewriting at all: the suggested parameter names ARE the
/// source variable names (em 02/03 promotion), so the moved statements bind
/// unchanged — every local/parameter read inside the selection is a parameter
/// of the new method, every variable declared inside moves with the body, and
/// the remaining reads are type members visible from the same class
/// (the new method is inserted into the SAME containing type). This is the
/// property the resolver (one method body, whole statements) and the
/// classifier (params bucket = exactly the local/parameter reads) jointly
/// guarantee; a generic SyntaxRewriter renaming pass would be dead weight in
/// v1. What IS generated structurally:
///   - the new method declaration (signature line, static-ness mirrored from
///     the enclosing method, body = selected statements + appended
///     <c>return ...</c> for return slots the selection did not already end
///     with a return of),
///   - the call statement(s) replacing the selection,
///   - the insertion of the new method right after the enclosing one.
///
/// CALL-SITE RULES (per promoted return slots, v1):
/// - void → a plain call expression statement. A ref parameter already flows
///   the value back (e.g. Accumulate: <c>Extract(ref acc, n);</c>).
/// - one slot DECLARED inside the selection → redeclared at the call site
///   from the result: <c>int total = Extract(limit);</c>.
/// - one slot declared OUTSIDE → plain assignment: <c>acc = Extract(...);</c>.
/// - several slots ALL declared inside → tuple deconstruction:
///   <c>var (x, y) = Extract(n);</c>.
/// - several slots, any declared outside → capture + ItemN assignments
///   (the result is evaluated ONCE, then assigned member-wise).
/// - the enclosing method's return type is non-void and the selection ends
///   the method → the call site must be <c>return Extract(...);</c> and the
///   suggested return type must equal the enclosing one (otherwise: refuse).
///
/// REFUSALS (loud, never a silent wrong refactoring):
/// - the selection contains a return that does NOT end the enclosing method
///   (v1: select to the end of the method);
/// - the suggested return type does not match the enclosing method's return
///   type — including the em 03 "composite trailing return" case, where the
///   signature is void although the enclosing method returns a value (the
///   report's note already says: extract a local first, then re-run).
///
/// BUILDER STYLE: single-slot declarations and assignments are hand-built
/// with SyntaxFactory; the deconstruction call <c>var (x, y) = ...;</c> and
/// the tuple return <c>return (x, y);</c> are PARSED from a text skeleton and
/// the call slot swapped in via ReplaceNode — the parser knows the
/// designation/tuple-element shapes a v1 builder would get wrong. (Parse
/// diagnostics would surface as red nodes and fail the round-trip check
/// loudly, not silently.)
///
/// KNOWN v1 APPROXIMATIONS: type display strings are re-parsed for the
/// generated declarations (a tuple return with a comma-bearing generic type
/// element is not supported); the capture name <c>result</c> could in theory
/// collide with an enclosing local; the name check "suggested return type ==
/// enclosing return type" is a string comparison, not symbol equality.
/// </summary>
public static class RefactoringGenerator
{
    public static Refactoring Generate(SemanticModel model, SelectionReport selection, SignatureSuggestion signature)
    {
        var method = selection.Method!;
        var body = method.Body!;

        var declaredInside = SignatureBuilder.DeclaredInsideNames(model, selection);
        var slots = signature.ReturnSlots.ToList();
        var slotNames = slots.Select(s => s.Name).ToList();

        var containsReturn = selection.Statements.OfType<ReturnStatementSyntax>().Any();
        var selectionEndsBody = ReferenceEquals(selection.Statements[^1], body.Statements[^1]);
        var enclosingNonVoid = method.ReturnType is not PredefinedTypeSyntax voidType
            || !voidType.Keyword.IsKind(SyntaxKind.VoidKeyword);
        var enclosingReturnType = method.ReturnType.ToString();

        // ---- soundness refusals (see class comment) ----
        if (containsReturn && enclosingNonVoid)
        {
            if (!selectionEndsBody)
            {
                throw new InvalidOperationException(
                    "the selection contains a return that does not end the enclosing method — select to the end of the method (v1 codegen)");
            }

            if (signature.ReturnType != enclosingReturnType)
            {
                throw new InvalidOperationException(
                    signature.ReturnType == "void"
                        ? "the selection ends with the method's return, but its value is not nameable in v1 — " +
                          "extract it into a local first (see the report note), then re-run"
                        : $"the suggested return type {signature.ReturnType} does not match the enclosing method's " +
                          $"return type {enclosingReturnType} — refusing to generate");
            }
        }

        // ---- the call expression (parameter names = source names, ref-ness kept) ----
        var arguments = signature.Params
            .Select(p =>
            {
                var argument = SyntaxFactory.Argument(SyntaxFactory.IdentifierName(p.Name));
                return p.ByRef
                    ? argument.WithRefOrOutKeyword(SyntaxFactory.Token(SyntaxKind.RefKeyword))
                    : argument;
            })
            .ToList();
        var invocation = SyntaxFactory.InvocationExpression(
                SyntaxFactory.IdentifierName(signature.MethodName))
            .WithArgumentList(SyntaxFactory.ArgumentList(SyntaxFactory.SeparatedList(arguments)))
            // Normalized once, here: the PARSED deconstruction skeleton (see
            // BuildCallStatements) must not be re-normalized (the normalizer
            // drops the space after the contextual keyword `var` before `(`),
            // so it receives the invocation already properly spaced.
            .NormalizeWhitespace();

        // Normalizing only the GENERATED nodes keeps the original trivia —
        // and therefore the doc comments — untouched (a whole-root
        // NormalizeWhitespace corrupts doc-comment trivia in a way Roslyn's
        // XML-doc writer rejects with CS1569, and it would also reformat the
        // entire file, making the printed diff useless). The call statements
        // and the new method get their indentation/end-of-line grafted from
        // the selection they replace, so the edit slots into the original
        // formatting.
        var callStatements = BuildCallStatements(slots, slotNames, invocation, declaredInside,
            callMustReturn: containsReturn && enclosingNonVoid).ToList();
        var indent = SyntaxFactory.TriviaList(
            selection.Statements[0].GetLeadingTrivia().Where(t => t.IsKind(SyntaxKind.WhitespaceTrivia)));
        for (var index = 0; index < callStatements.Count; index++)
        {
            var call = callStatements[index].WithLeadingTrivia(indent);
            if (index == callStatements.Count - 1)
            {
                // The replaced statement's trailing end-of-line kept the
                // following token (the next statement or the closing brace)
                // on its own line — carry it over.
                call = call.WithTrailingTrivia(selection.Statements[^1].GetTrailingTrivia());
            }

            callStatements[index] = call;
        }

        // Normalize the new method INSIDE a throwaway class wrapper: the
        // normalizer computes indentation from nesting depth, and a bare
        // method node has none — its body would come out flush-left. At class
        // depth the signature lands one level in and the body two. The grafted
        // leading/trailing trivia below replaces whatever the wrapper put
        // around the member.
        var newMethod = (MethodDeclarationSyntax)SyntaxFactory.ClassDeclaration("__normalizer__")
            .WithMembers(SyntaxFactory.SingletonList<MemberDeclarationSyntax>(
                BuildNewMethod(method, selection, signature, slots, slotNames)))
            .NormalizeWhitespace()
            .Members[0];
        var methodIndent = method.GetLeadingTrivia()
            .LastOrDefault(t => t.IsKind(SyntaxKind.WhitespaceTrivia));
        newMethod = newMethod
            .WithLeadingTrivia(SyntaxFactory.TriviaList(SyntaxFactory.EndOfLine("\n"), methodIndent))
            .WithTrailingTrivia(SyntaxFactory.EndOfLine("\n"));

        // ---- apply to the tree: swap the selection for the call, add the method ----
        var allStatements = body.Statements;
        var firstIndex = allStatements.IndexOf(selection.Statements[0]); // reference identity: same tree
        var newStatements = new List<StatementSyntax>();
        newStatements.AddRange(allStatements.Take(firstIndex));
        newStatements.AddRange(callStatements);
        newStatements.AddRange(allStatements.Skip(firstIndex + selection.Statements.Count));
        var rewrittenMethod = method.WithBody(body.WithStatements(SyntaxFactory.List(newStatements)));

        // The new method lands right after the enclosing one, in the SAME type
        // (that is what keeps the extract-first own-class members in scope).
        var containingType = method.AncestorsAndSelf().OfType<TypeDeclarationSyntax>().First();
        var memberIndex = containingType.Members.IndexOf(method);
        var newType = containingType.WithMembers(
            containingType.Members.Replace(method, rewrittenMethod).Insert(memberIndex + 1, newMethod));

        var tree = model.SyntaxTree;
        var visited = new SingleNodeReplacer(containingType, newType).Visit(tree.GetRoot());
        if (visited is null)
        {
            throw new InvalidOperationException("the type replacement rewrote the root away (internal error)");
        }

        return new Refactoring(newMethod, callStatements, tree.WithRootAndOptions(visited, tree.Options));
    }

    /// <summary>
    /// The one place a SyntaxRewriter enters this tool (yak em 04's stated
    /// curriculum): swapping a single node by REFERENCE identity. The typed
    /// With*-slices (WithBody, WithMembers) cover the local changes; only the
    /// "put the rewritten type back into the root" step is a whole-tree walk.
    /// </summary>
    private sealed class SingleNodeReplacer : CSharpSyntaxRewriter
    {
        private readonly SyntaxNode _oldNode;
        private readonly SyntaxNode _newNode;

        public SingleNodeReplacer(SyntaxNode oldNode, SyntaxNode newNode)
        {
            _oldNode = oldNode;
            _newNode = newNode;
        }

        public override SyntaxNode? Visit(SyntaxNode? node)
            => ReferenceEquals(node, _oldNode) ? _newNode : base.Visit(node);
    }

    /// <summary>The call statement(s) replacing the selection (see class comment).</summary>
    private static IReadOnlyList<StatementSyntax> BuildCallStatements(
        IReadOnlyList<ReturnSuggestion> slots,
        IReadOnlyList<string> slotNames,
        InvocationExpressionSyntax invocation,
        ISet<string> declaredInside,
        bool callMustReturn)
    {
        if (callMustReturn)
        {
            // The selection ends the enclosing (non-void) method: the call is
            // the return value, the slots live in the new method.
            return [Norm(SyntaxFactory.ReturnStatement(invocation))];
        }

        if (slots.Count == 0)
        {
            return [Norm(SyntaxFactory.ExpressionStatement(invocation))];
        }

        if (slots.Count == 1)
        {
            var slot = slots[0];
            if (declaredInside.Contains(slot.Name))
            {
                // Declared inside the selection: redeclare it at the call site
                // from the result (the declaration statement moved with the body).
                var declarator = SyntaxFactory.VariableDeclarator(slot.Name)
                    .WithInitializer(SyntaxFactory.EqualsValueClause(invocation));
                var declaration = SyntaxFactory.VariableDeclaration(
                    ParseType(slot.Type), SyntaxFactory.SingletonSeparatedList(declarator));
                return [Norm(SyntaxFactory.LocalDeclarationStatement(declaration))];
            }

            // Declared outside: the caller already owns it (a parameter, or a
            // local declared before the selection) — plain assignment.
            return [Norm(SyntaxFactory.ExpressionStatement(SyntaxFactory.AssignmentExpression(
                SyntaxKind.SimpleAssignmentExpression,
                SyntaxFactory.IdentifierName(slot.Name),
                invocation)))];
        }

        // Several return slots.
        if (slots.All(s => declaredInside.Contains(s.Name)))
        {
            // All declared inside: tuple deconstruction at the call site.
            // Parsed from a skeleton (see class comment — builder style), the
            // call swapped in at the placeholder identifier. NOT normalized:
            // the skeleton text is already well-spaced, and NormalizeWhitespace
            // would drop the space after the contextual keyword `var` (it is
            // an identifier token to the normalizer, so `var (` becomes
            // `var(`).
            return [ParseStatementWithCall($"var ({string.Join(", ", slotNames)}) = __call__;", invocation)];
        }

        // Mixed: evaluate the call ONCE into a capture, then assign each slot
        // from the matching ItemN. (Capture name collision with an enclosing
        // local is a documented v1 limitation.)
        var statements = new List<StatementSyntax>
        {
            Norm(SyntaxFactory.LocalDeclarationStatement(SyntaxFactory.VariableDeclaration(
                SyntaxFactory.PredefinedType(SyntaxFactory.Token(SyntaxKind.VarKeyword)),
                SyntaxFactory.SingletonSeparatedList(SyntaxFactory.VariableDeclarator("result")
                    .WithInitializer(SyntaxFactory.EqualsValueClause(invocation)))))),
        };
        for (var index = 0; index < slots.Count; index++)
        {
            statements.Add(Norm(SyntaxFactory.ExpressionStatement(SyntaxFactory.AssignmentExpression(
                SyntaxKind.SimpleAssignmentExpression,
                SyntaxFactory.IdentifierName(slots[index].Name),
                SyntaxFactory.MemberAccessExpression(SyntaxKind.SimpleMemberAccessExpression,
                    SyntaxFactory.IdentifierName("result"),
                    SyntaxFactory.IdentifierName($"Item{index + 1}"))))));
        }

        return statements;
    }

    /// <summary>
    /// Normalizes a builder-made statement: SyntaxFactory tokens carry no
    /// trivia, so without this <c>return</c> and <c>Extract</c> would render
    /// glued together. Only used for BUILDER statements — the parsed skeleton
    /// statements are already well-spaced (see the deconstruction comment).
    /// </summary>
    private static StatementSyntax Norm(StatementSyntax statement) => statement.NormalizeWhitespace();

    /// <summary>
    /// Parses <paramref name="statementText"/> (containing the placeholder
    /// identifier <c>__call__</c>) and swaps the placeholder for the built
    /// <paramref name="invocation"/> — the parser supplies the syntax shapes
    /// (tuple designations, tuple elements) the v1 builder avoids.
    /// </summary>
    private static StatementSyntax ParseStatementWithCall(string statementText, InvocationExpressionSyntax invocation)
    {
        var parsed = SyntaxFactory.ParseStatement(statementText);
        var placeholder = parsed.DescendantNodes()
            .OfType<IdentifierNameSyntax>()
            .First(n => n.Identifier.ValueText == "__call__");
        return parsed.ReplaceNode(placeholder, invocation);
    }

    /// <summary>
    /// The extracted method: the promoted signature, static-ness mirrored from
    /// the enclosing method, body = the selected statements plus an appended
    /// <c>return</c> for the return slots (skipped when the selection already
    /// ends in a return — a second one would be unreachable). The appended
    /// tuple return is parsed from its text skeleton (see class comment).
    /// </summary>
    private static MethodDeclarationSyntax BuildNewMethod(
        MethodDeclarationSyntax enclosingMethod,
        SelectionReport selection,
        SignatureSuggestion signature,
        IReadOnlyList<ReturnSuggestion> slots,
        IReadOnlyList<string> slotNames)
    {
        // A static enclosing method can only read statics and parameters, so
        // the extracted body needs no instance — mirror the static modifier.
        var modifiers = enclosingMethod.Modifiers.Any(m => m.IsKind(SyntaxKind.StaticKeyword))
            ? new[] { SyntaxFactory.Token(SyntaxKind.StaticKeyword) }
            : Array.Empty<SyntaxToken>();

        var parameters = signature.Params
            .Select(p =>
            {
                var parameter = SyntaxFactory.Parameter(SyntaxFactory.Identifier(p.Name))
                    .WithType(ParseType(p.Type));
                return p.ByRef
                    ? parameter.WithModifiers(SyntaxFactory.TokenList(
                        SyntaxFactory.Token(SyntaxKind.RefKeyword)))
                    : parameter;
            })
            .ToList();

        var bodyStatements = selection.Statements.ToList();
        if (slots.Count > 0 && selection.Statements[^1] is not ReturnStatementSyntax)
        {
            // The slot order here is the same name-ordered sequence the tuple
            // ReturnType was built from, so the emitted value matches the
            // declared element types.
            bodyStatements.Add(slots.Count == 1
                ? SyntaxFactory.ReturnStatement(SyntaxFactory.IdentifierName(slotNames[0]))
                : SyntaxFactory.ParseStatement($"return ({string.Join(", ", slotNames)});"));
        }

        return SyntaxFactory.MethodDeclaration(ParseType(signature.ReturnType), SyntaxFactory.Identifier(signature.MethodName))
            .WithModifiers(SyntaxFactory.TokenList(modifiers))
            .WithParameterList(SyntaxFactory.ParameterList(SyntaxFactory.SeparatedList(parameters)))
            .WithBody(SyntaxFactory.Block(SyntaxFactory.List(bodyStatements)));
    }

    /// <summary>
    /// Re-parses a type display string (em 02/03 are plain strings) in type
    /// context — <c>ParseTypeName</c> handles plain names, generics and tuple
    /// display strings <c>(T1, T2)</c> alike.
    ///
    /// EXCEPT <c>void</c>: parsing "void" in TYPE context yields a node the
    /// compiler rejects as a method return type with CS1547 (keyword 'void'
    /// cannot be used in this context) — the parser that reads real method
    /// declarations produces <c>PredefinedType(Token(VoidKeyword))</c>, so
    /// that is what a void signature must be built from.
    /// Single choke point for the string-to-syntax hop; its correctness is
    /// enforced by the round-trip check and the tests, not by construction.
    /// </summary>
    private static TypeSyntax ParseType(string text) => text == "void"
        ? SyntaxFactory.PredefinedType(SyntaxFactory.Token(SyntaxKind.VoidKeyword))
        : SyntaxFactory.ParseTypeName(text);
}