// Copyright 2021 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "src/transform/fold_trivial_single_use_lets.h"

#include "src/program_builder.h"
#include "src/sem/block_statement.h"
#include "src/sem/function.h"
#include "src/sem/statement.h"
#include "src/sem/variable.h"
#include "src/utils/scoped_assignment.h"

TINT_INSTANTIATE_TYPEINFO(tint::transform::FoldTrivialSingleUseLets);

namespace tint {
namespace transform {
namespace {

ast::VariableDeclStatement* AsTrivialLetDecl(ast::Statement* stmt) {
  auto* var_decl = stmt->As<ast::VariableDeclStatement>();
  if (!var_decl) {
    return nullptr;
  }
  auto* var = var_decl->variable();
  if (!var->is_const()) {
    return nullptr;
  }
  auto* ctor = var->constructor();
  if (!IsAnyOf<ast::IdentifierExpression, ast::ScalarConstructorExpression>(
          ctor)) {
    return nullptr;
  }
  return var_decl;
}

}  // namespace

FoldTrivialSingleUseLets::FoldTrivialSingleUseLets() = default;

FoldTrivialSingleUseLets::~FoldTrivialSingleUseLets() = default;

void FoldTrivialSingleUseLets::Run(CloneContext& ctx,
                                   const DataMap&,
                                   DataMap&) {
  for (auto* node : ctx.src->ASTNodes().Objects()) {
    if (auto* block = node->As<ast::BlockStatement>()) {
      auto& stmts = block->statements();
      for (size_t stmt_idx = 0; stmt_idx < stmts.size(); stmt_idx++) {
        auto* stmt = stmts[stmt_idx];
        if (auto* let_decl = AsTrivialLetDecl(stmt)) {
          auto* let = let_decl->variable();
          auto* sem_let = ctx.src->Sem().Get(let);
          auto& users = sem_let->Users();
          if (users.size() != 1) {
            continue;  // Does not have a single user.
          }

          auto* user = users[0];
          auto* user_stmt = user->Stmt()->Declaration();

          for (size_t i = stmt_idx; i < stmts.size(); i++) {
            if (user_stmt == stmts[i]) {
              auto* user_expr = user->Declaration();
              ctx.Remove(stmts, let_decl);
              ctx.Replace(user_expr, ctx.Clone(let->constructor()));
            }
            if (!AsTrivialLetDecl(stmts[i])) {
              // Stop if we hit a statement that isn't the single use of the
              // let, and isn't a let itself.
              break;
            }
          }
        }
      }
    }
  }

  ctx.Clone();
}

}  // namespace transform
}  // namespace tint