Automatically infer variables' measurement level from format and data.

[pspp] / doc / dev / system-file-format.texi

diff --git a/doc/dev/system-file-format.texi b/doc/dev/system-file-format.texi
index 500d74f66556fcff16d9ef14cf547a2375b8ee2e..476aec405a3e909a36f29547eee0879be3d2d607 100644 (file)
--- a/doc/dev/system-file-format.texi
+++ b/doc/dev/system-file-format.texi
@@ -1,3 +1,13 @@
+@c PSPP - a program for statistical analysis.
+@c Copyright (C) 2019 Free Software Foundation, Inc.
+@c Permission is granted to copy, distribute and/or modify this document
+@c under the terms of the GNU Free Documentation License, Version 1.3
+@c or any later version published by the Free Software Foundation;
+@c with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
+@c A copy of the license is included in the section entitled "GNU
+@c Free Documentation License".
+@c
+
 @node System File Format
 @appendix System File Format
 
@@ -6,7 +16,7 @@ that describes how they may be interpreted.  This chapter describes
 the format of a system file.
 
 System files use four data types: 8-bit characters, 32-bit integers,
-64-bit integers, 
+64-bit integers,
 and 64-bit floating points, called here @code{char}, @code{int32},
 @code{int64}, and
 @code{flt64}, respectively.  Data is not necessarily aligned on a word
@@ -548,7 +558,7 @@ The value label record has the following format:
 int32               rec_type;
 int32               label_count;
 
-/* @r{Repeated @code{label_cnt} times}. */
+/* @r{Repeated @code{n_label} times}. */
 char                value[8];
 char                label_len;
 char                label[];
@@ -991,18 +1001,24 @@ members are as follows:
 
 @table @code
 @item int32 measure;
-The measurement type of the variable:
+The measurement level of the variable:
 @table @asis
+@item 0
+Unknown
 @item 1
-Nominal Scale
+Nominal
 @item 2
-Ordinal Scale
+Ordinal
 @item 3
-Continuous Scale
+Scale
 @end table
 
-SPSS sometimes writes a @code{measure} of 0.  PSPP interprets this as
-nominal scale.
+An ``unknown'' @code{measure} of 0 means that the variable was created
+in some way that doesn't make the measurement level clear, e.g.@: with
+a @code{COMPUTE} transformation.  PSPP sets the measurement level the
+first time it reads the data using the rules documented in
+@ref{Measurement Level,,,pspp, PSPP Users Guide}, so this should
+rarely appear.
 
 @item int32 width;
 The width of the display column for the variable in characters.
@@ -1641,7 +1657,7 @@ The number of bytes in the ZLIB data trailer.  This and the previous
 field sum to the size of the system file in bytes.
 @end table
 
-The data header is followed by @code{(ztrailer_ofs - 24) / 24} ZLIB
+The data header is followed by @code{(ztrailer_len - 24) / 24} ZLIB
 compressed data blocks.  Each ZLIB compressed data block begins with a
 ZLIB header as specified in RFC@tie{}1950, e.g.@: hex bytes @code{78
 01} (the only header yet observed in practice).  Each block
@@ -1678,7 +1694,7 @@ been observed so far.
 
 @item int32 n_blocks;
 The number of ZLIB compressed data blocks, always exactly
-@code{(ztrailer_ofs - 24) / 24}.
+@code{(ztrailer_len - 24) / 24}.
 @end table
 
 The fixed header is followed by @code{n_blocks} 24-byte ZLIB data