diff --git a/Makefile b/Makefile
index ecf3071..491c3c7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 CFLAGS+=-g -O2 -march=native -Wall -Wextra -Wpedantic -std=c99 -Wformat=2 -Wconversion -Wtrampolines \
--Wimplicit-fallthrough
+-Wimplicit-fallthrough -D_XOPEN_SOURCE=500
 LIBS+=-lm
 
 all: bin/ bin/create-sample bin/analyze
diff --git a/README.md b/README.md
index 859dffc..9d87d44 100644
--- a/README.md
+++ b/README.md
@@ -25,9 +25,9 @@ This will create a 12 GB file with 1B rows named `measurements.txt` in your curr
 ### Run the challenge:
 
 ```
-time bin/analyze measurements.txt
+time bin/analyze measurements.txt >/dev/null
 
-real    0m29.071s
-user    0m23.790s
-sys     0m2.962s
+real    0m4.464s
+user    0m34.472s
+sys     0m6.938s
 ```
diff --git a/analyze.c b/analyze.c
index 83ea3d4..d5ad693 100644
--- a/analyze.c
+++ b/analyze.c
@@ -1,16 +1,13 @@
 #include <fcntl.h>
+#include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 #include <unistd.h>
 
-struct result {
-  char *city;
-  int count, sum, min, max;
-};
-
 #define HCAP 1987
 
 // parses a floating point number as an integer
@@ -28,8 +25,7 @@ static inline void parse_number(int *dest, char *s, char **endptr) {
 
   // parse characteristic
   while (*s >= '0' && *s <= '9') {
-    n = (n * 10) + (*s - '0');
-    s++;
+    n = (n * 10) + (*s++ - '0');
   }
 
   // skip separator
@@ -37,76 +33,75 @@ static inline void parse_number(int *dest, char *s, char **endptr) {
 
   // parse mantissa
   while (*s >= '0' && *s <= '9') {
-    d = (d * 10) + (*s - '0');
-    s++;
+    d = (d * 10) + (*s++ - '0');
   }
 
-  *dest = mod * (n * 10 + d);
+  *dest = mod * n * 10 + d;
   *endptr = s;
 }
 
 // hash returns the fnv-1a hash of the first n bytes in data
 static inline unsigned int hash(const unsigned char *data, size_t n) {
-  const unsigned int fnv_prime = 0x811C9DC5;
   unsigned int hash = 0;
 
   for (size_t i = 0; i < n; data++, i++) {
-    hash *= fnv_prime;
+    hash *= 0x811C9DC5; // prime
     hash ^= (*data);
   }
 
   return hash;
 }
 
-// cmp returns -1 if city property of a is lexically smaller than city property
-// of b
+struct Group {
+  unsigned int count;
+  int sum, min, max;
+  char *city;
+};
+
+// cmp compares the city property of the result that both pointers point
 static inline int cmp(const void *ptr_a, const void *ptr_b) {
-  return strcmp(((struct result *)ptr_a)->city, ((struct result *)ptr_b)->city);
+  return strcmp(((struct Group *)ptr_a)->city, ((struct Group *)ptr_b)->city);
 }
 
-int main(int argc, char **argv) {
-  char *file = "measurements.txt";
-  if (argc > 1) {
-    file = argv[1];
-  }
-
-  struct stat sb;
-  int fd = open(file, O_RDONLY);
-  if (!fd) {
-    perror("error opening file");
-    exit(EXIT_FAILURE);
-  }
+struct ResultSet {
+  char labels[420][32];
+  struct Group groups[420];
+  int n;
+};
 
-  if (fstat(fd, &sb) == -1) {
-    perror("error getting file size");
-    exit(EXIT_FAILURE);
-  }
+struct Chunk {
+  char *data;
+  size_t start;
+  size_t end;
+  struct ResultSet result;
+};
 
-  char cities[413][28] = {{0}};
-  struct result results[413];
-  int nresults = 0;
+static void *process_chunk(void *data) {
+  struct Chunk *ch = (struct Chunk *)data;
 
-  // mmap entire file into memory
-  char *data = mmap(NULL, (size_t)sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
-  if (!data) {
-    perror("error mmapping file");
-    exit(EXIT_FAILURE);
+  // skip start forward until SOF or after next newline
+  if (ch->start > 0) {
+    while (ch->data[ch->start - 1] != '\n') {
+      ch->start++;
+    }
   }
 
-  // initialize map to -1
-  int *map = malloc(HCAP * sizeof(int));
-  if (!map) {
-    perror("error allocating memory for hashmap");
-    exit(EXIT_FAILURE);
+  while (ch->data[ch->end] != 0x0 && ch->data[ch->end - 1] != '\n') {
+    ch->end++;
   }
+
+  // initialize hashmap
+  // will hold indexes into our cities and results array
+  int map[HCAP];
   memset(map, -1, HCAP * sizeof(int));
 
-  char *s = data;
+  char *s = &ch->data[ch->start];
   char *start, *pos;
   unsigned int h;
   int measurement;
   size_t len;
   int c;
+
   while (1) {
     start = s;
     pos = strchr(s, ';');
@@ -116,64 +111,125 @@ int main(int argc, char **argv) {
     // probe map until free spot or match
     h = hash((unsigned char *)start, len) % HCAP;
     c = map[h];
-    while (c >= 0 && memcmp(cities[c], start, len) != 0) {
-      h = (h == HCAP - 1) ? 0 : h + 1;
+    while (c >= 0 && memcmp(ch->result.labels[c], start, len) != 0) {
+      if (h++ == HCAP) {
+        h = 0;
+      }
       c = map[h];
     }
 
     if (c < 0) {
-      map[h] = nresults;
-      char *city = cities[nresults];
-      memcpy(city, start, len);
-      city[len] = 0x0;
-
-      results[nresults].city = city;
-      results[nresults].sum = measurement;
-      results[nresults].max = measurement;
-      results[nresults].min = measurement;
-      results[nresults].count = 1;
-
-      nresults++;
+      memcpy(ch->result.labels[ch->result.n], start, len);
+      ch->result.labels[ch->result.n][len] = 0x0;
+      ch->result.groups[ch->result.n].city = ch->result.labels[ch->result.n];
+      ch->result.groups[ch->result.n].count = 1;
+      ch->result.groups[ch->result.n].sum = measurement;
+      ch->result.groups[ch->result.n].min = measurement;
+      ch->result.groups[ch->result.n].max = measurement;
+      map[h] = ch->result.n++;
     } else {
-
-      results[c].sum += measurement;
-      results[c].count += 1;
-      if (measurement < results[c].min) {
-        results[c].min = measurement;
-      } else if (measurement > results[c].max) {
-        results[c].max = measurement;
+      ch->result.groups[c].count += 1;
+      ch->result.groups[c].sum += measurement;
+      if (measurement < ch->result.groups[c].min) {
+        ch->result.groups[c].min = measurement;
+      } else if (measurement > ch->result.groups[c].max) {
+        ch->result.groups[c].max = measurement;
       }
     }
 
     // skip newline
-    s++;
-
-    if (*s == 0x0) {
+    // break out of loop if EOF
+    if (*++s == 0x0 || s == &ch->data[ch->end]) {
       break;
     }
   }
 
-  char buf[128];
-  char *output = malloc(1 << 14 * sizeof(char));
-  if (!output) {
-    perror("error allocating memory for output string");
+  return (void *)ch;
+}
+
+int main(int argc, char **argv) {
+  char *file = "measurements.txt";
+  if (argc > 1) {
+    file = argv[1];
+  }
+
+  int fd = open(file, O_RDONLY);
+  if (!fd) {
+    perror("error opening file");
     exit(EXIT_FAILURE);
   }
 
-  s = output;
+  struct stat sb;
+  if (fstat(fd, &sb) == -1) {
+    perror("error getting file size");
+    exit(EXIT_FAILURE);
+  }
+
+  // mmap entire file into memory
+  size_t sz = (size_t)sb.st_size;
+  char *data = mmap(NULL, sz, PROT_READ, MAP_PRIVATE, fd, 0);
+  if (!data || data == MAP_FAILED) {
+    perror("error mmapping file");
+    exit(EXIT_FAILURE);
+  }
+
+  // distribute work among N worker threads
+  const int nworkers = 12;
+  pthread_t workers[nworkers];
+  struct Chunk chunks[nworkers];
+  for (int i = 0; i < nworkers; i++) {
+    chunks[i].result.n = 0;
+    chunks[i].data = data;
+    chunks[i].start = sz / (size_t)nworkers * (size_t)i;
+    chunks[i].end = sz / (size_t)nworkers * ((size_t)i + 1);
+    pthread_create(&workers[i], NULL, process_chunk, &chunks[i]);
+  }
+
+  // wait for all threads to finish
+  for (int i = 0; i < nworkers; i++) {
+    pthread_join(workers[i], NULL);
+  }
+
+  // merge results
+  struct ResultSet result = chunks[0].result;
+  for (int i = 1; i < nworkers; i++) {
+    for (int j = 0; j < chunks[i].result.n; j++) {
+      // get index of label
+      for (int k = 0; k < result.n; k++) {
+        if (strcmp(result.labels[k], chunks[i].result.labels[j]) == 0) {
+          result.groups[k].count += chunks[i].result.groups[j].count;
+          result.groups[k].sum += chunks[i].result.groups[j].sum;
+          if (chunks[i].result.groups[j].min < result.groups[k].min) {
+            result.groups[k].min = chunks[i].result.groups[j].min;
+          }
+          if (chunks[i].result.groups[j].max < result.groups[k].max) {
+            result.groups[k].max = chunks[i].result.groups[j].max;
+          }
+          break;
+        }
+      }
+    }
+  }
+
+  // sort results alphabetically
+  qsort(result.groups, (size_t)result.n, sizeof(struct Group), cmp);
+
+  // prepare output string
+  char buf[128];
+  char output[1 << 14];
+  char *s = output;
   *s++ = '{';
-  qsort(results, (size_t)nresults, sizeof(struct result), cmp);
-  for (int i = 0; i < nresults; i++) {
+  for (int i = 0; i < result.n; i++) {
     size_t n = (size_t)sprintf(
-        buf, "%s=%.1f/%.1f/%.1f", results[i].city,
-        (double)results[i].min / 10.0,
-        ((double)results[i].sum / (double)results[i].count) / 10.0,
-        (double)results[i].max / 10.0);
+        buf, "%s=%.1f/%.1f/%.1f", result.groups[i].city,
+        (float)result.groups[i].min / 10.0,
+        ((float)result.groups[i].sum / (float)result.groups[i].count) / 10.0,
+        (float)result.groups[i].max / 10.0);
 
     // copy buf to output
     memcpy(s, buf, n);
 
-    if (i < nresults - 1) {
+    if (i < result.n - 1) {
       memcpy(s + n, ", ", 2);
       n += 2;
     }
@@ -184,12 +240,7 @@ int main(int argc, char **argv) {
   *s = 0x0;
   puts(output);
 
-  free(map);
-  free(output);
-  munmap(data, sb.st_size);
+  munmap(data, sz);
   close(fd);
   exit(EXIT_SUCCESS);
 }
-
-// TODO: Concurrency, process in chunks where each chunk works on a separate map
-// TODO: SIMD