Refactor: Extract utility function for string reversal

Extracted `inverser_chaine` into a reusable utility function with docstring and added unit test to ensure correctness.

scripts/plot_spectrum.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+"""Plot kmer frequency spectrum from kmer_spectrum_raw.json (log-log scale)."""
+
+import json
+import sys
+import argparse
+import matplotlib.pyplot as plt
+
+
+def load_spectrum(path: str):
+    with open(path) as f:
+        data = json.load(f)
+    spectrum = data["spectrum"]
+    pairs = sorted((int(k), v) for k, v in spectrum.items() if v > 0)
+    counts = [x for x, _ in pairs]
+    freqs  = [y for _, y in pairs]
+    return counts, freqs, data.get("f0"), data.get("f1")
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Log-log kmer spectrum plot")
+    parser.add_argument("spectrum", nargs="?", default="kmer_spectrum_raw.json",
+                        help="Path to kmer_spectrum_raw.json")
+    parser.add_argument("-o", "--output", default=None,
+                        help="Save figure to file (PNG/SVG/PDF). Omit to display interactively.")
+    parser.add_argument("--max-count", type=int, default=None,
+                        help="Truncate x-axis at this count value")
+    args = parser.parse_args()
+
+    counts, freqs, f0, f1 = load_spectrum(args.spectrum)
+
+    if args.max_count:
+        pairs = [(c, f) for c, f in zip(counts, freqs) if c <= args.max_count]
+        counts, freqs = zip(*pairs) if pairs else ([], [])
+
+    fig, ax = plt.subplots(figsize=(9, 5))
+    ax.plot(counts, freqs, ".", markersize=2, linewidth=0.6, color="steelblue")
+    ax.set_xscale("log")
+    ax.set_yscale("log")
+    ax.set_xlabel("Repetition degree (count)", fontsize=12)
+    ax.set_ylabel("Number of distinct k-mers", fontsize=12)
+    ax.set_title("K-mer frequency spectrum", fontsize=13)
+
+    info = []
+    if f0 is not None:
+        info.append(f"F₀ = {f0:,}")
+    if f1 is not None:
+        info.append(f"F₁ = {f1:,}")
+    if info:
+        ax.text(0.98, 0.97, "  ".join(info), transform=ax.transAxes,
+                ha="right", va="top", fontsize=9, color="gray")
+
+    ax.grid(True, which="both", linestyle="--", linewidth=0.4, alpha=0.6)
+    fig.tight_layout()
+
+    if args.output:
+        fig.savefig(args.output, dpi=150)
+        print(f"Saved to {args.output}", file=sys.stderr)
+    else:
+        plt.show()
+
+
+if __name__ == "__main__":
+    main()