FUNCTION interp_coeffs_1d, vdata, poles, tol, DanIDL_C_code

; Description: This function calculates the vector of interpolation coefficients for an input vector
;              of data "vdata" and a given interpolation scheme with filter poles "poles". The 
;              calculation of the initial value of the causal coefficient can be speeded up by setting 
;              the value of "tol" to a positive value greater than "0.0" but less than "1.0", which 
;              represents the maximum acceptable relative error during the filtering process.
;                This function will use C++ code if possible to achieve a much faster execution than 
;              the default IDL code. The C++ code is an astounding factor of ~62 times faster than the 
;              IDL code in this case.
;
; Input Parameters:
;   
;   vdata - FLOAT/DOUBLE VECTOR - A one-dimensional data vector for which the interpolation coefficients 
;                                 are to be calculated. Note that this data vector should not contain 
;                                 bad data values, so such values should have been previously 
;                                 interpolated over in "vdata". 
;   poles - FLOAT/DOUBLE VECTOR - A one-dimensional vector of values for the poles required for the recursive
;                                 filter. A list of the pole values required for each interpolation scheme
;                                 are reported here for convenience:
;
;                                 Interpolation Scheme:  No. Poles:  First Pole:                             Second Pole:
;
;                                 Quadratic B-splines    1           sqrt(8)-3
;                                 Cubic B-Splines        1           sqrt(3)-2
;                                 Quartic B-Splines      2           sqrt(664-sqrt(438976))+sqrt(304)-19     sqrt(664+sqrt(438976))-sqrt(304)-19
;                                 Quintic B-Splines      2           (sqrt(270-sqrt(70980))+sqrt(105)-13)/2  (sqrt(270+sqrt(70980))-sqrt(105)-13)/2
;                                 Cubic O-MOMS           1           (sqrt(105)-13)/8
;
;   tol - FLOAT/DOUBLE - The maximum acceptable relative error during the filtering process. By setting 
;                        this parameter to a positive value greater than "0.0" but less than "1.0", the 
;                        initial value of the causal coefficient is calculated from a subset of data 
;                        values, while achieving the acceptable relative error "tol". By setting this 
;                        parameter to any other value, the initial value of the causal coefficient is 
;                        calculated exactly from all data values in the vector.
;   DanIDL_C_code - STRING - The full directory path indicating where the DanIDL C++ code is installed. 
;                            The shared library of DanIDL C++ routines should exist as 
;                            "<DanIDL_C_code>/dist/libDanIDL.so" within the installation.
;
; Return Value:
;
;   The return value is a one-dimensional vector of DOUBLES of the same length as "vdata" that 
;   represents the interpolation coefficients for the given interpolation scheme with filter poles 
;   "poles". If this module fails for any reason, then the return value is 0.0D.
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   15/08/2008 - Module created (dmb)  

;Check that "vdata" is a one-dimensional vector of the correct number type, and if there is only one
;input data point, then return the input data point

;Check that "poles" is a one-dimensional vector of the correct number type

;Check that "tol" is a number of the correct type

;Determine if the shared library of DanIDL C++ routines is installed

;Set up the output vector of interpolation coefficients

;If the shared library of DanIDL C++ routines is installed, then use the C++ code to speed up 
;the calculation of the interpolation coefficients

  ;Calculate the interpolation coefficients

;If the shared library of DanIDL C++ routines is not installed, then use the default IDL code

  ;Compute the overall gain, and apply this gain to the interpolation coefficients

  ;If "tol" is greater than "0.0" but less than "1.0", then calculate the lengths of the
  ;required truncated sums

  ;For each pole

    ;If the length of the truncated sum is less than the length of "vdata", then calculate
    ;the initial value of the causal coefficient with a truncated sum

    ;If the length of the truncated sum is the same as or greater than the length of "vdata", 
    ;then calculate the initial value of the causal coefficient with a full sum

    ;Execute the causal recursion on the interpolation coefficients

    ;Calculate the initial value of the anti-causal coefficient

    ;Execute the anti-causal recursion on the interpolation coefficients

;Return the output vector of interpolation coefficients