Number of Subarrays That Match a Pattern II — C++

// Time:  O(n)
// Space: O(m)

// kmp
class Solution {
public:
    int countMatchingSubarrays(vector<int>& nums, vector<int>& pattern) {
        const auto& KMP = [](const auto& text, const auto& pattern) {
            const auto& getPrefix = [](const auto& pattern) {
                vector<int> prefix(size(pattern), -1);
                int j = -1;
                for (int i = 1; i < size(pattern); ++i) {
                    while (j != -1 && pattern[j + 1] != pattern[i]) {
                        j = prefix[j];
                    }
                    if (pattern[j + 1] == pattern[i]) {
                        ++j;
                    }
                    prefix[i] = j;
                }
                return prefix;
            };

            int result = 0;
            const vector<int> prefix = getPrefix(pattern);
            int j = -1;
            for (int i = 0; i < size(text); ++i) {
                while (j > -1 && pattern[j + 1] != text[i]) {
                    j = prefix[j];
                }
                if (pattern[j + 1] == text[i]) {
                    ++j;
                }
                if (j == size(pattern) - 1) {
                    ++result;
                    j = prefix[j];
                }
            }
            return result;
        };
    
        for (int i = 0; i + 1 < size(nums); ++i) {
            if (nums[i + 1] > nums[i]) {
                nums[i] = 1;
            } else if (nums[i + 1] < nums[i]) {
                nums[i] = -1;
            } else {
                nums[i] = 0;
            }
        }
        return KMP(nums, pattern);
    }
};