//------------------------------------------------------------------------------
// <copyright file="StringConcat.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
// <owner current="true" primary="true">[....]</owner>
//------------------------------------------------------------------------------
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.ComponentModel;

namespace System.Xml.Xsl.Runtime {

    /// <summary>
    /// Efficiently concatenates strings when the number of string is not known beforehand, and
    /// yet it is common for only one string to be concatenated.  StringBuilder is not good for
    /// this purpose, since it *always* allocates objects, even if only one string is appended.
    /// </summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public struct StringConcat {
        private string s1, s2, s3, s4;
        private string delimiter;
        private List<string> strList;
        int idxStr;

        /// <summary>
        /// Clear the result string.
        /// </summary>
        public void Clear() {
            this.idxStr = 0;
            this.delimiter = null;
        }

        /// <summary>
        /// Gets or sets the string that delimits concatenated strings.
        /// </summary>
        public string Delimiter {
            get { return this.delimiter; }
            set { this.delimiter = value; }
        }

        /// <summary>
        /// Return the number of concatenated strings, including delimiters.
        /// </summary>
        internal int Count {
            get { return this.idxStr; }
        }

        /// <summary>
        /// Concatenate a new string to the result.
        /// </summary>
        public void Concat(string value) {
            Debug.Assert(value != null);

            if (this.delimiter != null && this.idxStr != 0) {
                // Add delimiter
                ConcatNoDelimiter(this.delimiter);
            }

            ConcatNoDelimiter(value);
        }

        /// <summary>
        /// Get the result string.
        /// </summary>
        public string GetResult() {
            switch (this.idxStr) {
                case 0: return string.Empty;
                case 1: return this.s1;
                case 2: return string.Concat(this.s1, this.s2);
                case 3: return string.Concat(this.s1, this.s2, this.s3);
                case 4: return string.Concat(this.s1, this.s2, this.s3, this.s4);
            }
            return string.Concat(this.strList.ToArray());
        }

        /// <summary>
        /// Concatenate a new string to the result without adding a delimiter.
        /// </summary>
        internal void ConcatNoDelimiter(string s) {
            switch (this.idxStr) {
                case 0: this.s1 = s; break;
                case 1: this.s2 = s; break;
                case 2: this.s3 = s; break;
                case 3: this.s4 = s; break;
                case 4:
                    // Calling Clear() is expensive, allocate a new List instead
                    int capacity = (this.strList == null) ? 8 : this.strList.Count;
                    List<string> strList = this.strList = new List<string>(capacity);
                    strList.Add(this.s1);
                    strList.Add(this.s2);
                    strList.Add(this.s3);
                    strList.Add(this.s4);
                    goto default;
                default:
                    this.strList.Add(s);
                    break;
            }

            this.idxStr++;
        }
    }
}