diff --git a/Makefile b/Makefile
index 17f68a4..0d09724 100644
--- a/Makefile
+++ b/Makefile
@@ -6,8 +6,12 @@ CXX = g++
 CXXFLAGS = -Wall -O2
   # -fsanitize=address -fsanitize=undefined
 
+.PHONY: all
+all: sortbin recgen
+
 sortbin: sortbin.cpp
+recgen: recgen.cpp
 
 .PHONY: clean
 clean:
-	$(RM) sortbin
+	$(RM) sortbin recgen
diff --git a/recgen.cpp b/recgen.cpp
new file mode 100644
index 0000000..89ce790
--- /dev/null
+++ b/recgen.cpp
@@ -0,0 +1,325 @@
+/*
+ * Tool to generate random binary data records.
+ *
+ * Written by Joris van Rantwijk in 2022.
+ */
+
+#define _FILE_OFFSET_BITS 64
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <math.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <vector>
+
+
+#define MAX_RECORD_SIZE             65536
+#define REPORT_INTERVAL             100000
+
+
+namespace {  // anonymous namespace
+
+
+class Xoroshiro128plus
+{
+public:
+    Xoroshiro128plus(uint64_t s0, uint64_t s1)
+    {
+        m_state[0] = s0;
+        m_state[1] = s1;
+    }
+
+    inline uint64_t next()
+    {
+        const uint64_t s0 = m_state[0];
+        uint64_t s1 = m_state[1];
+        const uint64_t result = s0 + s1;
+
+        s1 ^= s0;
+        m_state[0] = rotl(s0, 24) ^ s1 ^ (s1 << 16);
+        m_state[1] = rotl(s1, 37);
+
+        return result;
+    }
+
+private:
+    static inline uint64_t rotl(uint64_t x, int k)
+    {
+        return (x << k) | (x >> (64 - k));
+    }
+
+    uint64_t m_state[2];
+};
+
+
+
+class RecordGenerator
+{
+public:
+    RecordGenerator(
+        unsigned int record_size,
+        unsigned long long num_records,
+        double duplicate_fraction,
+        bool flag_ascii)
+      : m_record_size(record_size),
+        m_flag_ascii(flag_ascii)
+    {
+        assert(record_size > 0);
+
+        if (duplicate_fraction > 0) {
+            // Calculate the number of bits needed to achieve the
+            // specified duplicate fraction.
+            //
+            // This calculation is not exactly right, but it
+            // gives reasonable results for duplication_fraction > 0.5.
+            double num_values =
+                double(num_records) * (1 / duplicate_fraction - 1);
+            m_bits_per_record = lrint(ceil(log2(num_values)));
+        } else {
+            // Use uniform distribution of records.
+            m_bits_per_record = 8 * record_size;
+        }
+    }
+
+    void generate_record(unsigned char * record, Xoroshiro128plus& rng)
+    {
+        if (m_bits_per_record >= 8 * m_record_size || m_bits_per_record >= 128) {
+            // Just generate uniformly selected records.
+            // Nobody will notice the difference.
+            generate_uniform_record(record, rng);
+        } else {
+            // We have a budget of fewer than 128 random bits per record.
+            // Create a random seed value of that many bits, then use it
+            // to initialize a secondary random number generator to generate
+            // the data.
+            uint64_t s0 = 0, s1 = 0;
+            if (m_bits_per_record > 64) {
+                s0 = rng.next();
+                s1 = rng.next() >> (128 - m_bits_per_record);
+            } else {
+                s0 = rng.next() >> (64 - m_bits_per_record);
+                s1 = 0;
+            }
+            Xoroshiro128plus rng2(s0, s1);
+            rng2.next();
+            rng2.next();
+            generate_uniform_record(record, rng2);
+        }
+    }
+
+private:
+    void generate_uniform_record(unsigned char * record, Xoroshiro128plus& rng)
+    {
+        if (m_flag_ascii) {
+
+            // Generate ASCII record.
+            for (unsigned int i = 0; i < m_record_size - 1; i++) {
+                uint64_t r = rng.next() >> 4;
+                unsigned int p = r % 36;
+                if (p < 10) {
+                    record[i] = '0' + p;
+                } else {
+                    record[i] = 'a' + (p - 10);
+                }
+            }
+
+            // Append newline.
+            record[m_record_size-1] = '\n';
+
+        } else {
+
+            // Generate binary record.
+            for (unsigned int i = 0; i < m_record_size; i++) {
+                uint64_t r = rng.next() >> 4;
+                record[i] = r & 0xff;
+            }
+
+        }
+    }
+
+    unsigned int m_record_size;
+    unsigned int m_bits_per_record;
+    bool m_flag_ascii;
+};
+
+
+int recgen(
+    const char *output_name,
+    unsigned int record_size,
+    unsigned long long num_records,
+    double duplicate_fraction,
+    bool flag_ascii,
+    uint64_t seed)
+{
+    Xoroshiro128plus rng(seed, 0);
+    rng.next();
+    rng.next();
+
+    RecordGenerator record_generator(
+        record_size,
+        num_records,
+        duplicate_fraction,
+        flag_ascii);
+
+    int fd = open(output_name, O_WRONLY | O_CREAT | O_EXCL, 0666);
+    if (fd < 0) {
+        fprintf(stderr, "ERROR: Can not create output file (%s)\n",
+                strerror(errno));
+        return -1;
+    }
+
+    FILE *outf = fdopen(fd, "w");
+    if (outf == NULL) {
+        fprintf(stderr, "ERROR: fdopen() failed (%s)\n", strerror(errno));
+        close(fd);
+        return -1;
+    }
+
+    std::vector<unsigned char> record(record_size);
+
+    for (unsigned long long i = 0; i < num_records; i++) {
+        if ((i % REPORT_INTERVAL) == 0) {
+            printf("\rgenerated %llu / %llu records    ", i, num_records);
+            fflush(stdout);
+        }
+
+        record_generator.generate_record(record.data(), rng);
+
+        if (fwrite(record.data(), record_size, 1, outf) != 1) {
+            fprintf(stderr, "ERROR: Writing to output file failed (%s)\n",
+                    strerror(errno));
+            fclose(outf);
+            return -1;
+        }
+    }
+
+    printf("\rgenerated %llu records - done        \n", num_records);
+    fflush(stdout);
+
+    fclose(outf);
+
+    return 0;
+}
+
+
+void usage()
+{
+    fprintf(stderr,
+        "\n"
+        "Generate fixed-length random binary records.\n"
+        "\n"
+        "Usage: recgen [-a] [-d D] -n N -s S outputfile\n"
+        "\n"
+        "Options:\n"
+        "\n"
+        "  -a          generate ASCII records: 0-1, a-z, end in newline\n"
+        "  -d D        specify fraction of duplicate records (0.0 to 1.0)\n"
+        "  -n N        specify number of records (required)\n"
+        "  -s S        specify record size in bytes (required)\n"
+        "\n");
+}
+
+
+} // anonymous namespace
+
+
+int main(int argc, char **argv)
+{
+    double duplicate_fraction = 0.0;
+    unsigned long long num_records = 0;
+    unsigned long record_size = 0;
+    bool flag_ascii = false;
+    uint64_t seed = 1;
+    int opt;
+
+    while ((opt = getopt(argc, argv, "ad:n:s:")) != -1) {
+        char *endptr;
+        switch (opt) {
+            case 'a':
+                flag_ascii = true;
+                break;
+            case 'd':
+                duplicate_fraction = strtod(optarg, &endptr);
+                if (endptr == optarg
+                        || *endptr != '\0'
+                        || duplicate_fraction < 0.0
+                        || duplicate_fraction > 1.0) {
+                    fprintf(stderr,
+                        "ERROR: Invalid duplicate fraction"
+                        " (must be between 0.0 and 1.0)\n");
+                    return EXIT_FAILURE;
+                }
+                break;
+            case 'n':
+                num_records = strtoull(optarg, &endptr, 10);
+                if (endptr == optarg
+                        || *endptr != '\0'
+                        || num_records == 0) {
+                    fprintf(stderr,
+                        "ERROR: Invalid number of records\n");
+                    return EXIT_FAILURE;
+                }
+                break;
+            case 's':
+                record_size = strtoul(optarg, &endptr, 10);
+                if (endptr == optarg
+                        || *endptr != '\0'
+                        || record_size < 2
+                        || record_size > MAX_RECORD_SIZE) {
+                    fprintf(stderr,
+                        "ERROR: Invalid record size\n");
+                    return EXIT_FAILURE;
+                }
+                break;
+            case 'h':
+                usage();
+                return EXIT_SUCCESS;
+            default:
+                usage();
+                return EXIT_FAILURE;
+        }
+    }
+
+    if (num_records == 0) {
+        fprintf(stderr, "ERROR: Missing required parameter -n\n");
+        usage();
+        return EXIT_FAILURE;
+    }
+    if (record_size == 0) {
+        fprintf(stderr, "ERROR: Missing required parameter -s\n");
+        usage();
+        return EXIT_FAILURE;
+    }
+
+    if (argc < optind + 1) {
+        fprintf(stderr,
+                "ERROR: Output file name must be specified\n");
+        usage();
+        return EXIT_FAILURE;
+    }
+
+    if (argc > optind + 1) {
+        fprintf(stderr, "ERROR: Unexpected command-line parameters\n");
+        usage();
+        return EXIT_FAILURE;
+    }
+
+    const char * output_name = argv[optind];
+
+    int ret = recgen(
+        output_name,
+        record_size,
+        num_records,
+        duplicate_fraction,
+        flag_ascii,
+        seed);
+
+    return (ret == 0) ? EXIT_SUCCESS : EXIT_FAILURE;
+}