verify_hash_context.zig

/// This function issues a compile error with a helpful message if there
/// is a problem with the provided context type.  A context must have the following
/// member functions:
///   - hash(self, PseudoKey) Hash
///   - eql(self, PseudoKey, Key) bool
/// If you are passing a context to a *Adapted function, PseudoKey is the type
/// of the key parameter.  Otherwise, when creating a HashMap or HashMapUnmanaged
/// type, PseudoKey = Key = K.
pub fn verifyContext(comptime RawContext: type, comptime PseudoKey: type, comptime Key: type, comptime Hash: type) void {
    comptime {
        var allow_const_ptr = false;
        var allow_mutable_ptr = false;
        // Context is the actual namespace type.  RawContext may be a pointer to Context.
        var Context = RawContext;
        // Make sure the context is a namespace type which may have member functions
        switch (@typeInfo(Context)) {
            .Struct, .Union => {},
            // Special-case .Opaque for a better error message
            .Opaque => @compileError("Hash context must be a type with hash and eql member functions.  Cannot use "++@typeName(Context)++" because it is opaque.  Use a pointer instead."),
            .Pointer => |ptr| {
                if (ptr.size != .One) {
                    @compileError("Hash context must be a type with hash and eql member functions.  Cannot use "++@typeName(Context)++" because it is not a single pointer.");
                }
                Context = ptr.child;
                allow_const_ptr = true;
                allow_mutable_ptr = !ptr.is_const;
                switch (@typeInfo(Context)) {
                    .Struct, .Union, .Opaque => {},
                    else => @compileError("Hash context must be a type with hash and eql member functions.  Cannot use "++@typeName(Context)),
                }
            },
            else => @compileError("Hash context must be a type with hash and eql member functions.  Cannot use "++@typeName(Context)),
        }

        // Keep track of multiple errors so we can report them all.
        var errors: []const u8 = "";

        // Put common errors here, they will only be evaluated
        // if the error is actually triggered.
        const lazy = struct {
            const prefix = "\n  ";
            const deep_prefix = prefix ++ "  ";
            const hash_signature = "fn (self, "++@typeName(PseudoKey)++") "++@typeName(Hash);
            const eql_signature = "fn (self, "++@typeName(PseudoKey)++", "++@typeName(Key)++") bool";
            const err_invalid_hash_signature = prefix ++ @typeName(Context) ++ ".hash must be " ++ hash_signature ++
                    deep_prefix ++ "but is actually " ++ @typeName(@TypeOf(Context.hash));
            const err_invalid_eql_signature = prefix ++ @typeName(Context) ++ ".eql must be " ++ eql_signature ++
                    deep_prefix ++ "but is actually " ++ @typeName(@TypeOf(Context.eql));
        };

        // Verify Context.hash(self, PseudoKey) => Hash
        if (@hasDecl(Context, "hash")) {
            const hash = Context.hash;
            const info = @typeInfo(@TypeOf(hash));
            if (info == .Fn) {
                const func = info.Fn;
                if (func.args.len != 2) {
                    errors = errors ++ lazy.err_invalid_hash_signature;
                } else {
                    var emitted_signature = false;
                    if (func.args[0].arg_type) |Self| {
                        if (Self == Context) {
                            // pass, this is always fine.
                        } else if (Self == *const Context) {
                            if (!allow_const_ptr) {
                                if (!emitted_signature) {
                                    errors = errors ++ lazy.err_invalid_hash_signature;
                                    emitted_signature = true;
                                }
                                errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++", but is "++@typeName(Self);
                                errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be a pointer because it is passed by value.";
                            }
                        } else if (Self == *Context) {
                            if (!allow_mutable_ptr) {
                                if (!emitted_signature) {
                                    errors = errors ++ lazy.err_invalid_hash_signature;
                                    emitted_signature = true;
                                }
                                if (!allow_const_ptr) {
                                    errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++", but is "++@typeName(Self);
                                    errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be a pointer because it is passed by value.";
                                } else {
                                    errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++" or "++@typeName(*const Context)++", but is "++@typeName(Self);
                                    errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be non-const because it is passed by const pointer.";
                                }
                            }
                        } else {
                            if (!emitted_signature) {
                                errors = errors ++ lazy.err_invalid_hash_signature;
                                emitted_signature = true;
                            }
                            errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context);
                            if (allow_const_ptr) {
                                errors = errors++" or "++@typeName(*const Context);
                                if (allow_mutable_ptr) {
                                    errors = errors++" or "++@typeName(*Context);
                                }
                            }
                            errors = errors++", but is "++@typeName(Self);
                        }
                    }
                    if (func.args[1].arg_type != null and func.args[1].arg_type.? != PseudoKey) {
                        if (!emitted_signature) {
                            errors = errors ++ lazy.err_invalid_hash_signature;
                            emitted_signature = true;
                        }
                        errors = errors ++ lazy.deep_prefix ++ "Second parameter must be "++@typeName(PseudoKey)++", but is "++@typeName(func.args[1].arg_type.?);
                    }
                    if (func.return_type != null and func.return_type.? != Hash) {
                        if (!emitted_signature) {
                            errors = errors ++ lazy.err_invalid_hash_signature;
                            emitted_signature = true;
                        }
                        errors = errors ++ lazy.deep_prefix ++ "Return type must be "++@typeName(Hash)++", but was "++@typeName(func.return_type.?);
                    }
                    // If any of these are generic (null), we cannot verify them.
                    // The call sites check the return type, but cannot check the
                    // parameters.  This may cause compile errors with generic hash/eql functions.
                }
            } else {
                errors = errors ++ lazy.err_invalid_hash_signature;
            }
        } else {
            errors = errors ++ lazy.prefix ++ @typeName(Context) ++ " must declare a hash function with signature " ++ lazy.hash_signature;
        }

        // Verify Context.eql(self, PseudoKey, Key) => bool
        if (@hasDecl(Context, "eql")) {
            const eql = Context.eql;
            const info = @typeInfo(@TypeOf(eql));
            if (info == .Fn) {
                const func = info.Fn;
                if (func.args.len != 3) {
                    errors = errors ++ lazy.err_invalid_eql_signature;
                } else {
                    var emitted_signature = false;
                    if (func.args[0].arg_type) |Self| {
                        if (Self == Context) {
                            // pass, this is always fine.
                        } else if (Self == *const Context) {
                            if (!allow_const_ptr) {
                                if (!emitted_signature) {
                                    errors = errors ++ lazy.err_invalid_eql_signature;
                                    emitted_signature = true;
                                }
                                errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++", but is "++@typeName(Self);
                                errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be a pointer because it is passed by value.";
                            }
                        } else if (Self == *Context) {
                            if (!allow_mutable_ptr) {
                                if (!emitted_signature) {
                                    errors = errors ++ lazy.err_invalid_eql_signature;
                                    emitted_signature = true;
                                }
                                if (!allow_const_ptr) {
                                    errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++", but is "++@typeName(Self);
                                    errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be a pointer because it is passed by value.";
                                } else {
                                    errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context)++" or "++@typeName(*const Context)++", but is "++@typeName(Self);
                                    errors = errors ++ lazy.deep_prefix ++ "Note: Cannot be non-const because it is passed by const pointer.";
                                }
                            }
                        } else {
                            if (!emitted_signature) {
                                errors = errors ++ lazy.err_invalid_eql_signature;
                                emitted_signature = true;
                            }
                            errors = errors ++ lazy.deep_prefix ++ "First parameter must be "++@typeName(Context);
                            if (allow_const_ptr) {
                                errors = errors++" or "++@typeName(*const Context);
                                if (allow_mutable_ptr) {
                                    errors = errors++" or "++@typeName(*Context);
                                }
                            }
                            errors = errors++", but is "++@typeName(Self);
                        }
                    }
                    if (func.args[1].arg_type.? != PseudoKey) {
                        if (!emitted_signature) {
                            errors = errors ++ lazy.err_invalid_eql_signature;
                            emitted_signature = true;
                        }
                        errors = errors ++ lazy.deep_prefix ++ "Second parameter must be "++@typeName(PseudoKey)++", but is "++@typeName(func.args[1].arg_type.?);
                    }
                    if (func.args[2].arg_type.? != Key) {
                        if (!emitted_signature) {
                            errors = errors ++ lazy.err_invalid_eql_signature;
                            emitted_signature = true;
                        }
                        errors = errors ++ lazy.deep_prefix ++ "Third parameter must be "++@typeName(Key)++", but is "++@typeName(func.args[2].arg_type.?);
                    }
                    if (func.return_type.? != bool) {
                        if (!emitted_signature) {
                            errors = errors ++ lazy.err_invalid_eql_signature;
                            emitted_signature = true;
                        }
                        errors = errors ++ lazy.deep_prefix ++ "Return type must be bool, but was "++@typeName(func.return_type.?);
                    }
                    // If any of these are generic (null), we cannot verify them.
                    // The call sites check the return type, but cannot check the
                    // parameters.  This may cause compile errors with generic hash/eql functions.
                }
            } else {
                errors = errors ++ lazy.err_invalid_eql_signature;
            }
        } else {
            errors = errors ++ lazy.prefix ++ @typeName(Context) ++ " must declare a eql function with signature " ++ lazy.eql_signature;
        }

        if (errors.len != 0) {
            // errors begins with a newline (from lazy.prefix)
            @compileError("Problems found with hash context type "++@typeName(Context)++":"++errors);
        }
    }
}