From 06a6ded2e7582e3980e18dd79cfa687af23a8b39 Mon Sep 17 00:00:00 2001
From: norangebit <git@norangeb.it>
Date: Sun, 21 Mar 2021 14:01:03 +0100
Subject: [PATCH] Add comments

---
 main.c | 58 +++++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 45 insertions(+), 13 deletions(-)

diff --git a/main.c b/main.c
index e47ef9f..95a89d9 100644
--- a/main.c
+++ b/main.c
@@ -39,74 +39,106 @@ int main() {
     if (rank == MASTER)
         initialize();
 
+    // distribution of the length of the text portion to each process
     MPI_Scatter(text_piece, 1, MPI_INT, &private_text_len, 1, MPI_INT, MASTER, MPI_COMM_WORLD);
 
+    // allocation of space for text
     char *private_text = (char *) malloc(sizeof(char) * (private_text_len + 1));
 
-    MPI_Scatterv(text, text_piece, displacements, MPI_CHAR, private_text, private_text_len, MPI_CHAR, MASTER,
-                 MPI_COMM_WORLD);
+    // distribution of the text portion to each process
+    // scatterv is necessary because the last process receives a larger portion of text in case it is not perfectly divisible.
+    MPI_Scatterv(
+            text,
+            text_piece,
+            displacements,
+            MPI_CHAR,
+            private_text,
+            private_text_len,
+            MPI_CHAR, MASTER,
+            MPI_COMM_WORLD
+    );
     private_text[private_text_len] = '\0';
 
+    // sending the remain to the last trial
+    // this is necessary for the calculation of the offset
     if (rank == MASTER)
         MPI_Send(&remain, 1, MPI_INT, size - 1, DEFAULT_TAG, MPI_COMM_WORLD);
-    else if (rank == size -1)
+    else if (rank == size - 1)
         MPI_Recv(&remain, 1, MPI_INT, MASTER, DEFAULT_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
 
     printf("%d -> input: %s\n", rank, private_text);
     printf("%d -> text len: %d\n", rank, private_text_len);
-    printf("%d -> text len_: %d\n", rank, strlen(private_text));
 
+    // distribution of the length of the pattern to each process
     MPI_Bcast(&pattern_len, 1, MPI_INT, MASTER, MPI_COMM_WORLD);
 
+    // allocation of space for the pattern
+    // the master has already done so previously
     if (rank != MASTER) {
         pattern = (char *) malloc(sizeof(char) * (pattern_len + 1));
     }
 
+    // distribution of the pattern to each process
     MPI_Bcast(pattern, pattern_len + 1, MPI_CHAR, MASTER, MPI_COMM_WORLD);
 
+    // create and build lps array
     int lps[pattern_len];
-
     create_lps(pattern, pattern_len, lps);
 
     int match_number = 0, residue;
+    // search for matches
     int *matches = search_pattern(private_text, pattern, lps, &match_number, &residue);
 
-
+    // send the residue to the next process
     if (rank + 1 != size) {
         MPI_Send(&residue, 1, MPI_INT, rank + 1, DEFAULT_TAG, MPI_COMM_WORLD);
     }
 
+    // receiving the residue from the previous process
     if (rank != 0) {
         MPI_Recv(&residue, 1, MPI_INT, rank - 1, DEFAULT_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
         if (residue != 0)
+            // search for a split match
             find_end(residue, pattern, private_text, &matches, &match_number);
     }
 
-    //print_array(matches, match_number);
-
-    int shift = rank * (private_text_len - remain);
+    // transformation of match indices from relative to absolute position.
+    int shift = rank * (private_text_len - remain); // the rest is zero for all but the last process
     apply_shift(shift, matches, match_number);
 
     printf("%d -> result number: %d\n", rank, match_number);
 
     if (rank == MASTER) {
+        // allocation for the array containing the number of matches of each process
         match_numbers = (int *) malloc(sizeof(int) * size);
     }
 
+    // collection of the number of matches from each process
     MPI_Gather(&match_number, 1, MPI_INT, match_numbers, 1, MPI_INT, MASTER, MPI_COMM_WORLD);
 
     if (rank == MASTER) {
+        // preparation of the data structures needed to receive match indices from all processes
         total_match_number = sum_array(match_numbers, size);
-
         total_matches = (int *) malloc(sizeof(int) * total_match_number);
+
         displacements[0] = 0;
         for (int i = 0; i < size - 1; ++i) {
             displacements[i + 1] = displacements[i] + match_numbers[i];
         }
     }
 
-    MPI_Gatherv(matches, match_number, MPI_INT, total_matches, match_numbers, displacements, MPI_INT, MASTER,
-                MPI_COMM_WORLD);
+    // collection of match indices from each process
+    // gatherv is necessary because each process will have a different number of matches
+    MPI_Gatherv(
+            matches,
+            match_number,
+            MPI_INT,
+            total_matches,
+            match_numbers,
+            displacements,
+            MPI_INT, MASTER,
+            MPI_COMM_WORLD
+    );
 
     MPI_Finalize();
 
@@ -139,7 +171,7 @@ void find_end(int residue, char *pattern, char *text, int **matches, int *match_
 }
 
 void initialize() {
-    text = "questo è un test di esempio. Qui c'è la prima occorenza mamma e qui la seconda mamma";//read_file("data/text.txt", &text_len);
+    text = "testo di esempio mamma altra mamma esempio prova mammamma esempio mamma";//read_file("data/text.txt", &text_len);
     pattern = "mamma";//read_file("data/pattern.txt", &pattern_len);
 
     text_len = strlen(text);