// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.

package com.google.devtools.build.lib.unix;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.CharMatcher;
import com.google.common.collect.ImmutableMap;
import com.google.common.io.Files;
import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.util.List;
import java.util.Map;

/**
 * Parse and return information from /proc/meminfo.
 */
public class ProcMeminfoParser {

  public static final String FILE = "/proc/meminfo";

  private final Map<String, Long> memInfo;

  /**
   * Populates memory information by reading /proc/meminfo.
   * @throws IOException if reading the file failed.
   */
  public ProcMeminfoParser() throws IOException {
    this(FILE);
  }

  @VisibleForTesting
  public ProcMeminfoParser(String fileName) throws IOException {
    List<String> lines = Files.readLines(new File(fileName), Charset.defaultCharset());
    ImmutableMap.Builder<String, Long> builder = ImmutableMap.builder();
    for (String line : lines) {
      int colon = line.indexOf(':');
      if (colon == -1) {
        continue;
      }
      String keyword = line.substring(0, colon);
      String valString = line.substring(colon + 1);
      try {
        long val =  Long.parseLong(CharMatcher.inRange('0', '9').retainFrom(valString));
        builder.put(keyword, val);
      } catch (NumberFormatException e) {
        // Ignore: we'll fail later if somebody tries to capture this value.
      }
    }
    memInfo = builder.build();
  }

  /** Gets a named field in KB. */
  long getRamKb(String keyword) throws KeywordNotFoundException {
    Long val = memInfo.get(keyword);
    if (val == null) {
      throw new KeywordNotFoundException(keyword);
    }
    return val;
  }

  /** Return the total physical memory. */
  public long getTotalKb() throws KeywordNotFoundException {
    return getRamKb("MemTotal");
  }

  /**
   * Convert KB to MB.
   */
  public static double kbToMb(long kb) {
    return kb >> 10;
  }

  /**
   * Reads the amount of *available* memory as reported by the kernel. See https://goo.gl/ABn283 for
   * why this is better than trying to figure it out ourselves. This corresponds to the MemAvailable
   * line in /proc/meminfo.
   */
  public long getFreeRamKb() throws KeywordNotFoundException {
    if (memInfo.containsKey("MemAvailable")) {
      return getRamKb("MemAvailable");
    }
    // We have no MemAvailable in /proc/meminfo; fall back to the previous estimation.
    return getRamKb("MemTotal")
        - getRamKb("Active")
        // Blaze doesn't want to use more than a third of inactive ram...
        - (long) (getRamKb("Inactive") * 0.3)
        // ...and doesn't want to assume more than 80% of the slab memory can be reallocated.
        - (long) (getRamKb("Slab") * 0.8);
    // That said, this estimate will be more inaccurate as it diverges from kernel internals.
  }

  /** Exception thrown when /proc/meminfo does not have a requested key. Should be tolerated. */
  public static class KeywordNotFoundException extends Exception {
    private KeywordNotFoundException(String keyword) {
      super("Can't locate " + keyword + " in the /proc/meminfo");
    }
  }
}