diff --git a/pdf.c b/pdf.c
index b6e2089a4d77bd6376b50a97ccf1486352ed9435..cf3c3089808bbd8ef4d80d8ec80b72a2a5165020 100644
--- a/pdf.c
+++ b/pdf.c
@@ -244,6 +244,8 @@ act_a85zero(const HParseResult *p, void *u)
 	return H_MAKE_UINT(b);
 }
 
+#include <math.h>	/* pow() */
+
 HParsedToken *
 act_a85digit(const HParseResult *p, void *u)
 {
@@ -255,14 +257,13 @@ act_a85digit(const HParseResult *p, void *u)
 }
 
 HParsedToken *
-act_a85group(const HParseResult *p, void *u)
+act_a85fivedigits(const HParseResult *p, void *u)
 {
 	uint64_t fourbytes = 0;
 	HCountedArray *seq = H_CAST_SEQ(p->ast);
 	HParsedToken **digits = h_seq_elements(p->ast);
 
 	/* 2^32-1, the max value the group can hold as per spec */
-	// XXX test with "s8W-!"
 	#define A85GRPMAX 4294967295
 
 	/* Only for groups that do not need to padded to 5 */
@@ -274,12 +275,43 @@ act_a85group(const HParseResult *p, void *u)
 	fourbytes += H_CAST_UINT(digits[4]);
 
 	assert(fourbytes <= A85GRPMAX);
-	// XXX make it generic with H_ACT_APPLY
-	// XXX but partial groups are at least 2 characters long
-
 	return H_MAKE_UINT(fourbytes);
 }
 
+HParsedToken *
+act_a85group(const HParseResult *p, void *u)
+{
+
+}
+
+HParsedToken *
+act_a85partial2group(const HParseResult *p, void *u)
+{
+
+}
+
+HParsedToken *
+act_a85partial3group(const HParseResult *p, void *u)
+{
+
+}
+
+HParsedToken *
+act_a85partial4group(const HParseResult *p, void *u)
+{
+
+}
+
+/* Checking the following condition in the spec:
+ * The value represented by a group of 5 characters is greater than 2^32 - 1.
+*/
+bool
+validate_a85fivedigits(HParseResult *p, void *u)
+{
+	// XXX test with "s8W-!"
+	return H_CAST_UINT(p->ast) <= A85GRPMAX;
+}
+
 HParsedToken *
 act_nat(const HParseResult *p, void *u)
 {
@@ -1143,9 +1175,35 @@ ASCII85Decode(const Dict *parms, HBytes b, HParser *p)
 
 	H_RULE(a85eod,	SEQ(h_ch('~'), OPT(h_many(lwchar)), h_ch('>')));
 	H_ARULE(a85zero,	h_ch('z'));
+	H_ARULE(a85digit,	h_ch_range('!', 'u'));
 
-	fprintf(stderr, "ASCII85Decode: not implemented\n");
-	return NULL;
+	/* Line whitespace can occur between any digit and has to be ignored, */
+	/* Comments are not allowed inside streams, and % character should cause
+	 * a parse error. */
+	H_RULE(lws,	IGN(h_many(IGN(lwchar))));
+	#define MANY_LWS(X) h_many(CHX(lws, X))
+
+	/* This encoding of zero is not allowed */
+	H_RULE(a85fiveexcl, h_repeat_n(MANY_LWS(h_ch('!')), 5));
+	H_VARULE(a85fivedigits,	SEQ(h_and(h_not(a85fiveexcl)), h_repeat_n(MANY_LWS(a85digit), 5)));
+	//H_RULE(a85digitws, SEQ(a85digit, OPT(
+	H_ARULE(a85group,	CHX(a85zero, h_repeat_n(MANY_LWS(a85digit), 5)));
+	// XXX semantic actions need cleaning
+
+	H_ARULE(a85partial2group,	h_repeat_n(MANY_LWS(a85digit), 2));
+	H_ARULE(a85partial3group,	h_repeat_n(MANY_LWS(a85digit), 3));
+	H_ARULE(a85partial4group,	h_repeat_n(MANY_LWS(a85digit), 4));
+
+	H_RULE(a85string,	SEQ(h_many(a85group), OPT(CHX(a85partial2group, a85partial3group, a85partial4group)), a85eod));
+
+	res = h_parse(a85string, b.token, b.len);
+	if(!res)
+	{
+		fprintf(stderr, "parse error in ASCII85Decode filter\n");
+		return NULL;
+	}
+
+	return res;
 }
 
 /*