calculate melting temp of region

highlight a string of bases and calculate the Tm. source from primer3:


double oligoTm(char *dna, double DNA_nM = 50, double K_mM = 50)
{
   delta_H = dh * -100.0; /*
                             * Nearest-neighbor thermodynamic values for dh
                             * are given in 100 cal/mol of interaction.
                             */
    delta_S = ds * -0.1; /*
                              * Nearest-neighbor thermodynamic values for ds
                              * are in in .1 cal/K per mol of interaction.
                              */
    touppers(s);
    /* Use a finite-state machine (DFA) to calucluate dh and ds for s. */
    c = *s; s++;
    if (c == 'A') goto A_STATE;
    else if (c == 'G') goto G_STATE;
    else if (c == 'T') goto T_STATE;
    else if (c == 'C') goto C_STATE;
    else if (c == 'N') goto N_STATE;
    else goto ERROR;
    STATE(A);
    STATE(T);
    STATE(G);
    STATE(C);
    STATE(N);
    /*
     * See Rychlik, Spencer, Roads, Nucleic Acids Research, vol 18, no 21,
     * page 6410, eqn (ii).
     */
    return delta_H / (delta_S + 1.987 * log(DNA_nM/4000000000.0))
        - 273.15 + 16.6 * log10(K_mM/1000.0);
}