altos: Switch APRS to compressed position format

This provides lat/lon/alt in fewer bytes while improving precision.

Signed-off-by: Keith Packard <keithp@keithp.com>

3 files changed, 95 insertions, 0 deletions

diff --git a/src/math/ef_log.c b/src/math/ef_log.c
new file mode 100644
index 00000000..619fe909
--- /dev/null
+++ b/src/math/ef_log.c
@@ -0,0 +1,92 @@
+/* ef_log.c -- float version of e_log.c.
+ * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
+ */
+
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice 
+ * is preserved.
+ * ====================================================
+ */
+
+#include "fdlibm.h"
+
+#ifdef __STDC__
+static const float
+#else
+static float
+#endif
+ln2_hi =   6.9313812256e-01,	/* 0x3f317180 */
+ln2_lo =   9.0580006145e-06,	/* 0x3717f7d1 */
+two25 =    3.355443200e+07,	/* 0x4c000000 */
+Lg1 = 6.6666668653e-01,	/* 3F2AAAAB */
+Lg2 = 4.0000000596e-01,	/* 3ECCCCCD */
+Lg3 = 2.8571429849e-01, /* 3E924925 */
+Lg4 = 2.2222198546e-01, /* 3E638E29 */
+Lg5 = 1.8183572590e-01, /* 3E3A3325 */
+Lg6 = 1.5313838422e-01, /* 3E1CD04F */
+Lg7 = 1.4798198640e-01; /* 3E178897 */
+
+#ifdef __STDC__
+static const float zero   =  0.0;
+#else
+static float zero   =  0.0;
+#endif
+
+#ifdef __STDC__
+	float __ieee754_logf(float x)
+#else
+	float __ieee754_logf(x)
+	float x;
+#endif
+{
+	float hfsq,f,s,z,R,w,t1,t2,dk;
+	__int32_t k,ix,i,j;
+
+	GET_FLOAT_WORD(ix,x);
+
+	k=0;
+	if (FLT_UWORD_IS_ZERO(ix&0x7fffffff))
+	    return -two25/zero;		/* log(+-0)=-inf */
+        if (ix<0) return (x-x)/zero;	/* log(-#) = NaN */
+	if (!FLT_UWORD_IS_FINITE(ix)) return x+x;
+	if (FLT_UWORD_IS_SUBNORMAL(ix)) {
+	    k -= 25; x *= two25; /* subnormal number, scale up x */
+	    GET_FLOAT_WORD(ix,x);
+	} 
+	k += (ix>>23)-127;
+	ix &= 0x007fffff;
+	i = (ix+(0x95f64<<3))&0x800000;
+	SET_FLOAT_WORD(x,ix|(i^0x3f800000));	/* normalize x or x/2 */
+	k += (i>>23);
+	f = x-(float)1.0;
+	if((0x007fffff&(15+ix))<16) {	/* |f| < 2**-20 */
+           if(f==zero) { if(k==0) return zero;  else {dk=(float)k;
+                                return dk*ln2_hi+dk*ln2_lo;}}
+	    R = f*f*((float)0.5-(float)0.33333333333333333*f);
+	    if(k==0) return f-R; else {dk=(float)k;
+	    	     return dk*ln2_hi-((R-dk*ln2_lo)-f);}
+	}
+ 	s = f/((float)2.0+f); 
+	dk = (float)k;
+	z = s*s;
+	i = ix-(0x6147a<<3);
+	w = z*z;
+	j = (0x6b851<<3)-ix;
+	t1= w*(Lg2+w*(Lg4+w*Lg6)); 
+	t2= z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7))); 
+	i |= j;
+	R = t2+t1;
+	if(i>0) {
+	    hfsq=(float)0.5*f*f;
+	    if(k==0) return f-(hfsq-s*(hfsq+R)); else
+		     return dk*ln2_hi-((hfsq-(s*(hfsq+R)+dk*ln2_lo))-f);
+	} else {
+	    if(k==0) return f-s*(f-R); else
+		     return dk*ln2_hi-((s*(f-R)-dk*ln2_lo)-f);
+	}
+}
diff --git a/src/math/fdlibm.h b/src/math/fdlibm.h
index 821619ad..ee9fcb22 100644
--- a/src/math/fdlibm.h
+++ b/src/math/fdlibm.h
@@ -19,6 +19,7 @@
 
 #define __ieee754_acosf acosf
 #define __ieee754_sqrtf sqrtf
+#define __ieee754_logf logf
 
 /* REDHAT LOCAL: Include files.  */
 #include <math.h>
diff --git a/src/math/math.h b/src/math/math.h
index fd543bc2..97dd74cf 100644
--- a/src/math/math.h
+++ b/src/math/math.h
@@ -34,4 +34,6 @@ float scalbnf(float x, int n);
 
 float copysignf(float x, float y);
 
+float logf(float x);
+
 #endif