--summary

The summary tells you how much differs between two files before you write the diff.

rift --summary a.sql b.sql

a.sql, b.sql: a 64 MiB pair

$ rift --summary a.sql b.sql
a: 843633 lines, 67108879 bytes
b: 845034 lines, 67182379 bytes
stratum 0, 830263 anchors
equal    436 segments, 835632 lines in a, 835632 in b
differs  435 segments, 8001 lines in a, 9402 in b
first differing ranges (0-based, end exclusive):
  a 381..383  b 381..382
  a 1005..1011  b 1004..1010
  a 1280..1281  b 1279..1281
  a 8109..8119  b 8109..8110
  a 9618..9619  b 9609..9618
  a 11782..11783  b 11781..11790
  a 17463..17784  b 17470..17471
  a 21556..21560  b 21243..21244
  a 22749..22750  b 22433..22437
  a 25037..25048  b 24724..24725
  ... and 425 more
moved    62 blocks, 2737 lines
  a 21557..21560  b 22434..22437
  a 25038..25048  b 26389..26399
  a 27627..27952  b 28206..28531
  a 36864..36874  b 36680..36690
  a 48226..48235  b 49457..49466
  a 50776..50780  b 50896..50900
  a 59441..59445  b 61396..61400
  a 65493..65499  b 65178..65184
  a 99590..99595  b 99830..99835
  a 110203..110209  b 111213..111219
  ... and 52 more
peak heap 78.6 MiB of 256.0 MiB (82397510 bytes)
time 0.13 s

Read it from the top:

Counts. The line and byte count of each file.
Segments. Rift cuts the pair into segments at lines that occur once in each file. equal segments have identical bytes on both sides, checked byte by byte. differs segments hold the changes. The summary does not diff them line by line, so it cannot tell which ones would come out approximate; run the full diff with --stats for that count.
Ranges. The first 10 differing ranges, counted from 0, end excluded. a 381..383 b 381..382 means lines 381 and 382 of the first file became line 381 of the second.
Moved blocks. Blocks of at least three lines that appear in both files at different places. See moved blocks.
Resource use. The peak heap against the limit, and the time.

The summary writes no diff, so it is the cheapest run. On a 1 GiB pair it finished in 0.51 s on the Linux machine on the benchmarks page.

Use --summary in a script to decide whether a diff is worth writing, or to see how many segments will need refining. The exit status follows diff: 1 when the files differ.