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Hashing

Source: R/hash.R
hash.Rd

  • hash() hashes an arbitrary R object.

  • hash_file() hashes the data contained in a file.

For ordinary data objects (vectors, lists, data frames, etc.), the generated hash is reproducible across sessions and R versions on platforms that have the same endianness. However, hash values may change between rlang versions, although that should be rare. Reference-like objects (environments, external pointers, builtins) are hashed by identity, so their hashes are only stable within a session. Closures hash their formals, body, and environment identity. Byte-compiled and uncompiled closures hash identically because the body is always hashed from the original language tree.

By default, source references are stripped before hashing so that closures and calls that are textually identical produce the same hash regardless of where they were parsed. Set zap_srcref to FALSE to include source references in the hash.

Usage

hash(x, zap_srcref = TRUE)

hash_file(path)

Arguments

x

An object.

zap_srcref

Whether to ignore source references when hashing (default TRUE). Source references depend on parse location, so including them makes hashes of closures and calls non-reproducible across sessions.

path

A character vector of paths to the files to be hashed.

Value

  • For hash(), a single character string containing the hash.

  • For hash_file(), a character vector containing one hash per file.

Details

These hashers use the XXH128 hash algorithm of the xxHash library, which generates a 128-bit hash. Both are implemented as streaming hashes, which generate the hash with minimal extra memory usage.

For hash(), a custom object walker feeds the object's type, length, data bytes, and attributes directly into the hash algorithm. This avoids dependency on R's serialization format, making the hash immune to internal representation details (e.g. ALTREP compact forms, the growable vector bit).

Examples

hash(c(1, 2, 3))
#> [1] "7f210cbfa5470651f6f9e9d220634266"
hash(mtcars)
#> [1] "6755d143ff87b73a1196c186cec7e86a"

authors <- file.path(R.home("doc"), "AUTHORS")
copying <- file.path(R.home("doc"), "COPYING")
hashes <- hash_file(c(authors, copying))
hashes
#> [1] "91719ee600dd3908734502d8098b2e98"
#> [2] "cdb3a24318136e74f38209c219ca104b"

# If you need a single hash for multiple files,
# hash the result of `hash_file()`
hash(hashes)
#> [1] "548e3499d61c72e7508c7355a678963b"