1. A method for determining program equivalence between a first program and multiple candidate programs, the method comprising steps of: defining a program semantic representation for the first program; generating a search query based on said program semantic representation; executing said search query against an index of multiple programs, wherein said index comprises a program semantic representation for each of the multiple programs; outputting a set of candidate programs, among the multiple programs, as a result of executing said search query against the index; refactoring (i) the first program and (ii) each program in the set of candidate programs, wherein said refactoring is based on (a) one or more patterns of program equivalence and (b) one or more canonicalization operators; outputting a canonical representation of (i) the first program and (ii) each respective program in the set of candidate programs as a result of said refactoring; identifying one or more programs among the set of candidate programs that match the first program by performing one or more comparisons, across the set of candidate programs, of the canonical representation associated with each respective program in the identified set of candidate programs to the canonical representation associated with the first program, wherein said performing the one or more comparisons comprises: extracting multiple semantic features from (i) the canonical representation associated with each respective program in the identified set of candidate programs and (ii) the canonical representation associated with the first program; creating an index based on the extracted semantic features from the canonical representation associated with each respective program in the identified set of candidate programs; searching the created index using the extracted semantic features from the canonical representation associated with the first program; and computing similarity measures between (i) the canonical representation associated with each respective program in the identified set of candidate programs and (ii) the canonical representation associated with the first program, wherein each similarity measure comprises a function of overlap of extracted semantic features based on said searching, and wherein computing each similarity measure comprises: identifying a variable in a canonical representation associated with a given program in the set of candidate programs, wherein the identified variable is equivalent to an output variable in the canonical representation associated with the first program, and wherein said identifying comprises implementing data-slicing analysis; and analyzing slices with respect to (i) the identified variable in the canonical representation associated with a given program in the set of candidate programs and (ii) the output variable in the canonical representation associated with the first program to determining whether the given program and the first program both comply with a given pattern of computing; and outputting the one or more identified programs that match the first program to at least one user; wherein the steps are carried out by at least one processor coupled to a memory.