Extract metrics package from gerrit-server top-level directory

Change-Id: I7c21b82de289ebc24b530d6b37abf6c33b4dec2f

java/com/google/gerrit/metrics/Field.java

@@ -0,0 +1,170 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// 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.gerrit.metrics;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import com.google.common.base.Function;
+import com.google.common.base.Functions;
+
+/**
+ * Describes a bucketing field used by a metric.
+ *
+ * @param <T> type of field
+ */
+public class Field<T> {
+  /**
+   * Break down metrics by boolean true/false.
+   *
+   * @param name field name
+   * @return boolean field
+   */
+  public static Field<Boolean> ofBoolean(String name) {
+    return ofBoolean(name, null);
+  }
+
+  /**
+   * Break down metrics by boolean true/false.
+   *
+   * @param name field name
+   * @param description field description
+   * @return boolean field
+   */
+  public static Field<Boolean> ofBoolean(String name, String description) {
+    return new Field<>(name, Boolean.class, description);
+  }
+
+  /**
+   * Break down metrics by cases of an enum.
+   *
+   * @param enumType type of enum
+   * @param name field name
+   * @return enum field
+   */
+  public static <E extends Enum<E>> Field<E> ofEnum(Class<E> enumType, String name) {
+    return ofEnum(enumType, name, null);
+  }
+
+  /**
+   * Break down metrics by cases of an enum.
+   *
+   * @param enumType type of enum
+   * @param name field name
+   * @param description field description
+   * @return enum field
+   */
+  public static <E extends Enum<E>> Field<E> ofEnum(
+      Class<E> enumType, String name, String description) {
+    return new Field<>(name, enumType, description);
+  }
+
+  /**
+   * Break down metrics by string.
+   *
+   * <p>Each unique string will allocate a new submetric. <b>Do not use user content as a field
+   * value</b> as field values are never reclaimed.
+   *
+   * @param name field name
+   * @return string field
+   */
+  public static Field<String> ofString(String name) {
+    return ofString(name, null);
+  }
+
+  /**
+   * Break down metrics by string.
+   *
+   * <p>Each unique string will allocate a new submetric. <b>Do not use user content as a field
+   * value</b> as field values are never reclaimed.
+   *
+   * @param name field name
+   * @param description field description
+   * @return string field
+   */
+  public static Field<String> ofString(String name, String description) {
+    return new Field<>(name, String.class, description);
+  }
+
+  /**
+   * Break down metrics by integer.
+   *
+   * <p>Each unique integer will allocate a new submetric. <b>Do not use user content as a field
+   * value</b> as field values are never reclaimed.
+   *
+   * @param name field name
+   * @return integer field
+   */
+  public static Field<Integer> ofInteger(String name) {
+    return ofInteger(name, null);
+  }
+
+  /**
+   * Break down metrics by integer.
+   *
+   * <p>Each unique integer will allocate a new submetric. <b>Do not use user content as a field
+   * value</b> as field values are never reclaimed.
+   *
+   * @param name field name
+   * @param description field description
+   * @return integer field
+   */
+  public static Field<Integer> ofInteger(String name, String description) {
+    return new Field<>(name, Integer.class, description);
+  }
+
+  private final String name;
+  private final Class<T> keyType;
+  private final Function<T, String> formatter;
+  private final String description;
+
+  private Field(String name, Class<T> keyType, String description) {
+    checkArgument(name.matches("^[a-z_]+$"), "name must match [a-z_]");
+    this.name = name;
+    this.keyType = keyType;
+    this.formatter = initFormatter(keyType);
+    this.description = description;
+  }
+
+  /** @return name of this field within the metric. */
+  public String getName() {
+    return name;
+  }
+
+  /** @return type of value used within the field. */
+  public Class<T> getType() {
+    return keyType;
+  }
+
+  /** @return description text for the field explaining its range of values. */
+  public String getDescription() {
+    return description;
+  }
+
+  public Function<T, String> formatter() {
+    return formatter;
+  }
+
+  @SuppressWarnings("unchecked")
+  private static <T> Function<T, String> initFormatter(Class<T> keyType) {
+    if (keyType == String.class) {
+      return (Function<T, String>) Functions.<String>identity();
+    } else if (keyType == Integer.class || keyType == Boolean.class) {
+      return (Function<T, String>) Functions.toStringFunction();
+    } else if (Enum.class.isAssignableFrom(keyType)) {
+      return in -> ((Enum<?>) in).name();
+    }
+    throw new IllegalStateException("unsupported type " + keyType.getName());
+  }
+}