mirror of https://gerrit.osmocom.org/libosmocore
87c3afb5a9
This will be useful to handle latitude and longitude numbers for GAD, which is the location estimate representation used for LCS (Location Services). The OsmoSMLC VTY user interface will provide floating-point strings like "23.456" while GAD stores them as micro-degress 23456000. The osmo_gad_to_str* will also convert latitude and longitude to floating-point string. There was code review concerns against adding this API, upon which I tried to use floating point string formats. But I encountered various problems with accuracy and trailing zeros. For global positioning data (latitude and longitude), even inaccuracy on the sixth significant decimal digit causes noticeable positional shift. To achieve sufficient accuracy on the least significant end, I need to use double instead of float. To remove trailing zeros, the idea was to use '%.6g' format, but that can cause rounding. '%.6f' on a double looks ok, but always includes trailing zeros. A test program shows: %.6g of ((double)(int32_t)23230100)/1e6 = "23.2301" <-- good %.6g of ((double)(int32_t)42419993)/1e6 = "42.42" <-- bad rounding %.6g of ((double)(int32_t)23230199)/1e6 = "23.2302" <-- bad rounding %.6f of ((double)(int32_t)23230100)/1e6 = "23.230100" <-- trailing zeros %.6f of ((double)(int32_t)42419993)/1e6 = "42.419993" <-- good %.6f of ((double)(int32_t)23230199)/1e6 = "23.230199" <-- good It looks like when accepting that there will be trailing zeros, using double with '%.6f' would work out, but in the end I am not certain enough that there aren't more hidden rounding / precision glitches. Hence I decided to reinforce the need to add this API: it is glitch free in sufficient precision for latitude and longitude data, because it is based on integer arithmetic. The need for this precision is particular to the (new) OsmoSMLC vty configuration, where reading and writing back user config must not modify the values the user entered. Considering to add these functions to osmo-smlc.git, we might as well add them here to libosmocore utils, and also use them in osmo_gad_to_str_*() functions. Change-Id: Ib9aee749cd331712a4dcdadfb6a2dfa4c26da957 |
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| utils_test.c | ||
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