Question

I want an efficient algorithm to find all occurrences of a pattern within a larger sequence.

For example, given the following input:
Pattern: GAS
Sequence: ASDFGASDFGASDFADFASDFGA

Expected Output: {4, 9}

According to accepted answer to a similar question implements an algorithm for achieving the desired task. However, one comment reports the algorithm is “slow on large bytes array”.

After reading around, it appears the best algorithm for doing this is the Boyer-Moore String search algrorithm with an implementation in C# on CodeProject but I’m having trouble implementing it for generic enumerables.

Is there any existing solution based on the Boyer-Moore algorithm to find all occurrences of a pattern in a generic sequence in .NET?

Note
Though I used strings in my example I want an answer that works on any data that implements IEnumerable. In other words it should work not only on strings but on any type at all.

Answer 1

After struggling in vain to comprehend the Boyer-Moore algorithm, I put together this code which does the pattern matching with a single pass over the larger collection.

I have not been able to test it against the Boyer-Moore algorithm but it works quite efficiently, with O(nm) as worst-case performance when the whole sequence is a repetition of the pattern.

Here is my implementation. Let me know your views on it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter the string you want to search within.");
            string hayStack = Console.ReadLine();
            Console.WriteLine("Enter the string you want to search for.");
            string needle = Console.ReadLine();

            var ps = new PatternSearch<char>(needle.ToCharArray());

            Console.WriteLine();
            Console.WriteLine();

            Console.WriteLine(hayStack);

            var matches = ps.Matches(hayStack.ToCharArray()).ToList();

            for (int i = 0; i < hayStack.Length; i++)
                Console.Write(matches.Contains(i) ? "↑" : " ");

            Console.WriteLine();

            Console.ReadLine();
        }
    }

    /// <summary>Implements a pattern searching algorithm with <b>O(nm)</b> worst-case performance.</summary>
    /// <typeparam name="T">The data type of the array to search.</typeparam>
    public class PatternSearch<T>
    {
        private struct MatchInfo
        {
            public MatchInfo(int startIndex, int matchLength)
            {
                this.StartIndex = startIndex;
                this.MatchLength = matchLength;
            }
            public int StartIndex;
            public int MatchLength;
        }

        private IEnumerable<T> pattern;
        private List<MatchInfo> found;
        private Func<T, T, bool> eqComp;

        //optimization for IEnumerables that do not implement IList
        int patLen = -1;
        int seqLen = -1;

        /// <summary>Initializes a new instance of the <see cref="PatternSearch{T}" /> class.</summary>
        /// <param name="pattern">The pattern that will be searched for.</param>
        public PatternSearch(T[] pattern) : this(pattern, (x, y) => x.Equals(y)) { }

        /// <summary>
        /// Initializes a new instance of the <see cref="PatternSearch{T}"/> class with the specified equality comparer.
        /// </summary>
        /// <param name="pattern">The pattern to be searched for.</param>
        /// <param name="equalityComparer">The equality comparer to use for matching elements in the array.</param>
        public PatternSearch(T[] pattern, Func<T, T, bool> equalityComparer)
        {
            patLen = pattern.Length;

            if (pattern == null)
                throw new ArgumentNullException("pattern", "The search pattern cannot be null.");
            if (equalityComparer == null)
                throw new ArgumentNullException("equalityComparer", "The equality comparer cannot be null.");

            if (patLen <= 0)
                throw new ArgumentException("pattern", "The pattern cannot be empty.");

            // assign the values
            this.pattern = pattern;
            found = new List<MatchInfo>();
            eqComp = equalityComparer;
        }

        /// <summary>
        /// Returns the start index of all occurrences of the search pattern within the specified array.
        /// </summary>
        /// <param name="seq">The larger sequence to find occurrences of the search pattern within.</param>
        public IEnumerable<int> Matches(IEnumerable<T> seq)
        {
            seqLen = seqLen == -1 ? seq.Count() : seqLen;
            return this.Matches(seq, 0, seqLen);
        }

        /// <summary>
        /// Returns the start index of all occurrences of the search pattern within the specified array.
        /// </summary>
        /// <param name="seq">The larger sequence to find occurrences of the search pattern within.</param>
        /// <param name="startIndex">The index in <paramref name="seq"/> to start searching at.</param>
        public IEnumerable<int> Matches(IEnumerable<T> seq, int startIndex)
        {
            seqLen = seqLen == -1 ? seq.Count() : seqLen;
            return this.Matches(seq, startIndex, seqLen);
        }

        /// <summary>
        /// Returns the start index of all occurrences of the search pattern within the specified array.
        /// </summary>
        /// <param name="seq">The larger sequence to find occurrences of the search pattern within.</param>
        /// <param name="count">The maximum number of items in <paramref name="seq"/> to match.</param>
        public IEnumerable<int> Matches(IEnumerable<T> seq, int startIndex, int count)
        {
            patLen = patLen == -1 ? pattern.Count() : patLen;
            seqLen = seqLen == -1 ? seq.Count() : seqLen;
            bool addedNew = false;

            var endPoint = Math.Min(seqLen, startIndex + count);

            if (seq == null ||                      // sequence cannot be null
                seqLen < patLen ||                  // pattern cannot be longer than sequence
                (endPoint - startIndex) < patLen)   // start to end cannot be less than pattern
                yield break;

            for (int i = startIndex; i < endPoint; i++)
            {
                addedNew = false;

                // add the first item if a match is found
                if (eqComp(seq.ElementAt(i), pattern.ElementAt(0)))
                {
                    if (patLen == 1)
                        yield return i;

                    found.Add(new MatchInfo(i, 1));
                    addedNew = true;
                }

                // check incomplete matches
                for (int m = found.Count - 1; m >= 0; m--)
                {
                    //skip the last item added
                    if (addedNew && m == found.Count - 1)
                        continue;

                    var match = found[m];

                    // check incomplete matches
                    if ((i - match.StartIndex < patLen) &&
                        eqComp(seq.ElementAt(i), pattern.ElementAt(match.MatchLength)))
                    {
                        match.MatchLength += 1;
                        found[m] = match;

                        // determine if a complete match has been found
                        if (match.MatchLength == patLen)
                        {
                            yield return match.StartIndex;
                            found.RemoveAt(m);
                        }
                    }
                    else
                        found.RemoveAt(m);
                }
            }
        }

    }
}