[console] Fix definition of unreachability for remapped keys

The AltGr remapping table is constructed to include only keys that are
not reachable after applying the basic remapping table.  The logic
currently fails to include keys that are omitted entirely from the
basic remapping table since they would map to a non-ASCII character.

Fix this logic by allowing the remapping tables to include null
mappings, which are then elided only at the point of constructing the
C code fragment.

Reported-by: Christian Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

19 files changed, 137 insertions, 23 deletions

diff --git a/src/hci/keymap/keymap_al.c b/src/hci/keymap/keymap_al.c
index 4d279171f..6e14395a5 100644
--- a/src/hci/keymap/keymap_al.c
+++ b/src/hci/keymap/keymap_al.c
@@ -37,10 +37,15 @@ static struct keymap_key al_basic[] = {
 
 /** "al" AltGr remapping */
 static struct keymap_key al_altgr[] = {
+	{ 0x21, 0x7e },	/* '!' => '~' */
+	{ 0x26, 0x60 },	/* '&' => '`' */
 	{ 0x30, 0x7e },	/* '0' => '~' */
 	{ 0x31, 0x7e },	/* '1' => '~' */
 	{ 0x34, 0x7e },	/* '4' => '~' */
 	{ 0x37, 0x60 },	/* '7' => '`' */
+	{ 0x3a, 0x7e },	/* ':' => '~' */
+	{ 0x56, 0x60 },	/* 'V' => '`' */
+	{ 0x7c, 0x7e },	/* '|' => '~' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_cf.c b/src/hci/keymap/keymap_cf.c
index 434e3383c..55bbfc1a4 100644
--- a/src/hci/keymap/keymap_cf.c
+++ b/src/hci/keymap/keymap_cf.c
@@ -33,6 +33,7 @@ static struct keymap_key cf_altgr[] = {
 	{ 0x27, 0x7b },	/* '\'' => '{' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
 	{ 0x3b, 0x7e },	/* ';' => '~' */
+	{ 0x5c, 0x7d },	/* '\\' => '}' */
 	{ 0x60, 0x5c },	/* '`' => '\\' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_cz.c b/src/hci/keymap/keymap_cz.c
index fb072b117..08e9638e1 100644
--- a/src/hci/keymap/keymap_cz.c
+++ b/src/hci/keymap/keymap_cz.c
@@ -50,6 +50,9 @@ static struct keymap_key cz_basic[] = {
 
 /** "cz" AltGr remapping */
 static struct keymap_key cz_altgr[] = {
+	{ 0x21, 0x7e },	/* '!' => '~' */
+	{ 0x24, 0x7e },	/* '$' => '~' */
+	{ 0x29, 0x7e },	/* ')' => '~' */
 	{ 0x2c, 0x3c },	/* ',' => '<' */
 	{ 0x2e, 0x3e },	/* '.' => '>' */
 	{ 0x2f, 0x2a },	/* '/' => '*' */
@@ -60,10 +63,15 @@ static struct keymap_key cz_altgr[] = {
 	{ 0x37, 0x26 },	/* '7' => '&' */
 	{ 0x38, 0x2a },	/* '8' => '*' */
 	{ 0x39, 0x7b },	/* '9' => '{' */
+	{ 0x3a, 0x7e },	/* ':' => '~' */
 	{ 0x3b, 0x24 },	/* ';' => '$' */
+	{ 0x4b, 0x26 },	/* 'K' => '&' */
+	{ 0x58, 0x3e },	/* 'X' => '>' */
+	{ 0x5a, 0x3c },	/* 'Z' => '<' */
 	{ 0x61, 0x7e },	/* 'a' => '~' */
 	{ 0x62, 0x7b },	/* 'b' => '{' */
 	{ 0x63, 0x26 },	/* 'c' => '&' */
+	{ 0x66, 0x5b },	/* 'f' => '[' */
 	{ 0x67, 0x5d },	/* 'g' => ']' */
 	{ 0x6e, 0x7d },	/* 'n' => '}' */
 	{ 0x76, 0x40 },	/* 'v' => '@' */
diff --git a/src/hci/keymap/keymap_de.c b/src/hci/keymap/keymap_de.c
index bbd39520f..ca5e4a9a9 100644
--- a/src/hci/keymap/keymap_de.c
+++ b/src/hci/keymap/keymap_de.c
@@ -47,8 +47,13 @@ static struct keymap_key de_basic[] = {
 static struct keymap_key de_altgr[] = {
 	{ 0x2d, 0x5c },	/* '-' => '\\' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
 	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x3b, 0x7e },	/* ';' => '~' */
+	{ 0x5d, 0x7e },	/* ']' => '~' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
+	{ 0x7c, 0x7e },	/* '|' => '~' */
 	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_dk.c b/src/hci/keymap/keymap_dk.c
index 6d2e60aca..5b229024a 100644
--- a/src/hci/keymap/keymap_dk.c
+++ b/src/hci/keymap/keymap_dk.c
@@ -39,7 +39,14 @@ static struct keymap_key dk_basic[] = {
 static struct keymap_key dk_altgr[] = {
 	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x34, 0x24 },	/* '4' => '$' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x3a, 0x7e },	/* ':' => '~' */
 	{ 0x3d, 0x7c },	/* '=' => '|' */
+	{ 0x5c, 0x7e },	/* '\\' => '~' */
+	{ 0x5d, 0x7e },	/* ']' => '~' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_es.c b/src/hci/keymap/keymap_es.c
index 614ca7b8b..93d87f2a0 100644
--- a/src/hci/keymap/keymap_es.c
+++ b/src/hci/keymap/keymap_es.c
@@ -37,14 +37,27 @@ static struct keymap_key es_basic[] = {
 
 /** "es" AltGr remapping */
 static struct keymap_key es_altgr[] = {
+	{ 0x22, 0x7b },	/* '"' => '{' */
 	{ 0x27, 0x7b },	/* '\'' => '{' */
+	{ 0x2b, 0x7e },	/* '+' => '~' */
+	{ 0x2d, 0x5c },	/* '-' => '\\' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
+	{ 0x31, 0x7c },	/* '1' => '|' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x33, 0x23 },	/* '3' => '#' */
+	{ 0x34, 0x7e },	/* '4' => '~' */
 	{ 0x37, 0x7b },	/* '7' => '{' */
 	{ 0x38, 0x5b },	/* '8' => '[' */
 	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x3a, 0x7e },	/* ':' => '~' */
+	{ 0x3b, 0x7e },	/* ';' => '~' */
+	{ 0x3d, 0x7e },	/* '=' => '~' */
 	{ 0x5c, 0x7d },	/* '\\' => '}' */
+	{ 0x60, 0x5c },	/* '`' => '\\' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
+	{ 0x7c, 0x7e },	/* '|' => '~' */
+	{ 0x7e, 0x5c },	/* '~' => '\\' */
+	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_et.c b/src/hci/keymap/keymap_et.c
index f3eb02eab..4d763266f 100644
--- a/src/hci/keymap/keymap_et.c
+++ b/src/hci/keymap/keymap_et.c
@@ -35,8 +35,18 @@ static struct keymap_key et_basic[] = {
 
 /** "et" AltGr remapping */
 static struct keymap_key et_altgr[] = {
+	{ 0x26, 0x7b },	/* '&' => '{' */
+	{ 0x28, 0x5d },	/* '(' => ']' */
+	{ 0x29, 0x7d },	/* ')' => '}' */
+	{ 0x2a, 0x5b },	/* '*' => '[' */
 	{ 0x2d, 0x5c },	/* '-' => '\\' */
+	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x34, 0x24 },	/* '4' => '$' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x5f, 0x5c },	/* '_' => '\\' */
 	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_fi.c b/src/hci/keymap/keymap_fi.c
index 622f06252..ac69813e8 100644
--- a/src/hci/keymap/keymap_fi.c
+++ b/src/hci/keymap/keymap_fi.c
@@ -38,6 +38,12 @@ static struct keymap_key fi_altgr[] = {
 	{ 0x2d, 0x5c },	/* '-' => '\\' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x34, 0x24 },	/* '4' => '$' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x49, 0x7c },	/* 'I' => '|' */
+	{ 0x5d, 0x7e },	/* ']' => '~' */
 	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_fr.c b/src/hci/keymap/keymap_fr.c
index c002bf95a..ad797f876 100644
--- a/src/hci/keymap/keymap_fr.c
+++ b/src/hci/keymap/keymap_fr.c
@@ -68,9 +68,15 @@ static struct keymap_key fr_altgr[] = {
 	{ 0x2d, 0x5d },	/* '-' => ']' */
 	{ 0x30, 0x40 },	/* '0' => '@' */
 	{ 0x33, 0x23 },	/* '3' => '#' */
+	{ 0x34, 0x7b },	/* '4' => '{' */
 	{ 0x35, 0x5b },	/* '5' => '[' */
+	{ 0x36, 0x7c },	/* '6' => '|' */
+	{ 0x37, 0x60 },	/* '7' => '`' */
 	{ 0x38, 0x5c },	/* '8' => '\\' */
+	{ 0x3d, 0x7d },	/* '=' => '}' */
+	{ 0x5c, 0x60 },	/* '\\' => '`' */
 	{ 0x61, 0x40 },	/* 'a' => '@' */
+	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_hu.c b/src/hci/keymap/keymap_hu.c
index 8f2d70aac..d64baaebf 100644
--- a/src/hci/keymap/keymap_hu.c
+++ b/src/hci/keymap/keymap_hu.c
@@ -37,18 +37,36 @@ static struct keymap_key hu_basic[] = {
 
 /** "hu" AltGr remapping */
 static struct keymap_key hu_altgr[] = {
+	{ 0x21, 0x7e },	/* '!' => '~' */
+	{ 0x23, 0x5e },	/* '#' => '^' */
+	{ 0x26, 0x60 },	/* '&' => '`' */
+	{ 0x2c, 0x3b },	/* ',' => ';' */
 	{ 0x2e, 0x3e },	/* '.' => '>' */
 	{ 0x2f, 0x2a },	/* '/' => '*' */
+	{ 0x30, 0x7e },	/* '0' => '~' */
+	{ 0x31, 0x7e },	/* '1' => '~' */
 	{ 0x32, 0x5e },	/* '2' => '^' */
 	{ 0x33, 0x5e },	/* '3' => '^' */
+	{ 0x34, 0x7e },	/* '4' => '~' */
 	{ 0x37, 0x60 },	/* '7' => '`' */
 	{ 0x3b, 0x24 },	/* ';' => '$' */
+	{ 0x4b, 0x26 },	/* 'K' => '&' */
+	{ 0x58, 0x3e },	/* 'X' => '>' */
+	{ 0x5a, 0x3c },	/* 'Z' => '<' */
+	{ 0x62, 0x7b },	/* 'b' => '{' */
 	{ 0x63, 0x26 },	/* 'c' => '&' */
+	{ 0x66, 0x5b },	/* 'f' => '[' */
+	{ 0x67, 0x5d },	/* 'g' => ']' */
 	{ 0x6d, 0x3c },	/* 'm' => '<' */
+	{ 0x6e, 0x7d },	/* 'n' => '}' */
+	{ 0x71, 0x5c },	/* 'q' => '\\' */
 	{ 0x76, 0x40 },	/* 'v' => '@' */
+	{ 0x77, 0x7c },	/* 'w' => '|' */
 	{ 0x78, 0x23 },	/* 'x' => '#' */
 	{ 0x7a, 0x3e },	/* 'z' => '>' */
+	{ 0x7c, 0x7e },	/* '|' => '~' */
 	{ 0xdc, 0x3c },	/* Pseudo-'\\' => '<' */
+	{ 0xfc, 0x3e },	/* Pseudo-'|' => '>' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_it.c b/src/hci/keymap/keymap_it.c
index a55dafe6a..2511bb791 100644
--- a/src/hci/keymap/keymap_it.c
+++ b/src/hci/keymap/keymap_it.c
@@ -37,13 +37,19 @@ static struct keymap_key it_basic[] = {
 
 /** "it" AltGr remapping */
 static struct keymap_key it_altgr[] = {
+	{ 0x23, 0x7e },	/* '#' => '~' */
+	{ 0x27, 0x23 },	/* '\'' => '#' */
 	{ 0x2d, 0x60 },	/* '-' => '`' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
 	{ 0x39, 0x5d },	/* '9' => ']' */
 	{ 0x3b, 0x40 },	/* ';' => '@' */
 	{ 0x3d, 0x7e },	/* '=' => '~' */
+	{ 0x40, 0x7e },	/* '@' => '~' */
 	{ 0x5c, 0x60 },	/* '\\' => '`' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
+	{ 0x7c, 0x7e },	/* '|' => '~' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_lt.c b/src/hci/keymap/keymap_lt.c
index 5d6ee5a8c..a237b1d62 100644
--- a/src/hci/keymap/keymap_lt.c
+++ b/src/hci/keymap/keymap_lt.c
@@ -17,6 +17,10 @@ static struct keymap_key lt_basic[] = {
 
 /** "lt" AltGr remapping */
 static struct keymap_key lt_altgr[] = {
+	{ 0x22, 0x5e },	/* '"' => '^' */
+	{ 0x27, 0x5e },	/* '\'' => '^' */
+	{ 0x4b, 0x26 },	/* 'K' => '&' */
+	{ 0x71, 0x40 },	/* 'q' => '@' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_mt.c b/src/hci/keymap/keymap_mt.c
index 0a9a110b0..a95c5b4e8 100644
--- a/src/hci/keymap/keymap_mt.c
+++ b/src/hci/keymap/keymap_mt.c
@@ -22,6 +22,11 @@ static struct keymap_key mt_basic[] = {
 
 /** "mt" AltGr remapping */
 static struct keymap_key mt_altgr[] = {
+	{ 0x30, 0x7d },	/* '0' => '}' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x5c, 0x60 },	/* '\\' => '`' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_no-latin1.c b/src/hci/keymap/keymap_no-latin1.c
index 63fe85548..1f4cc38d3 100644
--- a/src/hci/keymap/keymap_no-latin1.c
+++ b/src/hci/keymap/keymap_no-latin1.c
@@ -39,10 +39,15 @@ static struct keymap_key no_latin1_basic[] = {
 
 /** "no-latin1" AltGr remapping */
 static struct keymap_key no_latin1_altgr[] = {
+	{ 0x22, 0x5b },	/* '"' => '[' */
+	{ 0x27, 0x7b },	/* '\'' => '{' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
 	{ 0x39, 0x5d },	/* '9' => ']' */
 	{ 0x5b, 0x7d },	/* '[' => '}' */
+	{ 0x7b, 0x5d },	/* '{' => ']' */
 	{ 0, 0 }
 };
 
diff --git a/src/hci/keymap/keymap_no.c b/src/hci/keymap/keymap_no.c
index 4b2f7268e..239e3aea5 100644
--- a/src/hci/keymap/keymap_no.c
+++ b/src/hci/keymap/keymap_no.c
@@ -41,6 +41,13 @@ static struct keymap_key no_basic[] = {
 static struct keymap_key no_altgr[] = {
 	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x34, 0x24 },	/* '4' => '$' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x3a, 0x7e },	/* ':' => '~' */
+	{ 0x5c, 0x7e },	/* '\\' => '~' */
+	{ 0x5d, 0x7e },	/* ']' => '~' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_pt.c b/src/hci/keymap/keymap_pt.c
index f294e9563..75bf565ce 100644
--- a/src/hci/keymap/keymap_pt.c
+++ b/src/hci/keymap/keymap_pt.c
@@ -44,6 +44,7 @@ static struct keymap_key pt_altgr[] = {
 	{ 0x32, 0x40 },	/* '2' => '@' */
 	{ 0x37, 0x7b },	/* '7' => '{' */
 	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
 	{ 0, 0 }
 };
diff --git a/src/hci/keymap/keymap_se.c b/src/hci/keymap/keymap_se.c
index 6ca990444..9c6cf64a4 100644
--- a/src/hci/keymap/keymap_se.c
+++ b/src/hci/keymap/keymap_se.c
@@ -40,6 +40,11 @@ static struct keymap_key se_altgr[] = {
 	{ 0x2d, 0x5c },	/* '-' => '\\' */
 	{ 0x30, 0x7d },	/* '0' => '}' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
+	{ 0x34, 0x24 },	/* '4' => '$' */
+	{ 0x37, 0x7b },	/* '7' => '{' */
+	{ 0x38, 0x5b },	/* '8' => '[' */
+	{ 0x39, 0x5d },	/* '9' => ']' */
+	{ 0x5d, 0x7e },	/* ']' => '~' */
 	{ 0x71, 0x40 },	/* 'q' => '@' */
 	{ 0xdc, 0x7c },	/* Pseudo-'\\' => '|' */
 	{ 0, 0 }
diff --git a/src/hci/keymap/keymap_sg.c b/src/hci/keymap/keymap_sg.c
index 9a6db9cb4..2afe59c61 100644
--- a/src/hci/keymap/keymap_sg.c
+++ b/src/hci/keymap/keymap_sg.c
@@ -45,9 +45,11 @@ static struct keymap_key sg_basic[] = {
 
 /** "sg" AltGr remapping */
 static struct keymap_key sg_altgr[] = {
+	{ 0x27, 0x7b },	/* '\'' => '{' */
 	{ 0x32, 0x40 },	/* '2' => '@' */
 	{ 0x33, 0x23 },	/* '3' => '#' */
 	{ 0x37, 0x7c },	/* '7' => '|' */
+	{ 0x3d, 0x7e },	/* '=' => '~' */
 	{ 0x5c, 0x7d },	/* '\\' => '}' */
 	{ 0, 0 }
 };
diff --git a/src/util/genkeymap.py b/src/util/genkeymap.py
index 8df949612..632f71eda 100755
--- a/src/util/genkeymap.py
+++ b/src/util/genkeymap.py
@@ -33,6 +33,10 @@ import textwrap
 from typing import ClassVar, Optional
 
 
+BACKSPACE = chr(0x7f)
+"""Backspace character"""
+
+
 class KeyType(IntEnum):
     """Key types"""
 
@@ -174,7 +178,7 @@ class KeyLayout(UserDict[KeyModifiers, Sequence[Key]]):
             KeyModifiers.NONE: [(KEY_NON_US, ord('\\'))],
             KeyModifiers.SHIFT: [(KEY_NON_US, ord('|'))],
             # Treat Ctrl-Backspace as producing Backspace rather than Ctrl-H
-            KeyModifiers.CTRL: [(KEY_BACKSPACE, 0x7f)],
+            KeyModifiers.CTRL: [(KEY_BACKSPACE, ord(BACKSPACE))],
         },
         'il': {
             # Redefine some otherwise unreachable ASCII characters
@@ -200,11 +204,6 @@ class KeyLayout(UserDict[KeyModifiers, Sequence[Key]]):
         """Basic shifted keyboard layout"""
         return self[KeyModifiers.SHIFT]
 
-    @property
-    def altgr(self):
-        """AltGr keyboard layout"""
-        return self.get(KeyModifiers.ALTGR, self.unshifted)
-
     @classmethod
     def load(cls, name: str) -> KeyLayout:
         """Load keymap using 'loadkeys -b'"""
@@ -281,7 +280,7 @@ class BiosKeyLayout(KeyLayout):
         return inverse
 
 
-class KeymapKeys(UserDict[str, str]):
+class KeymapKeys(UserDict[str, Optional[str]]):
     """An ASCII character remapping"""
 
     @classmethod
@@ -312,7 +311,8 @@ class KeymapKeys(UserDict[str, str]):
                 self.ascii_name(source), self.ascii_name(target)
             )
             for source, target in self.items()
-            if ord(source) & ~BiosKeyLayout.KEY_PSEUDO != ord(target)
+            if target
+            and ord(source) & ~BiosKeyLayout.KEY_PSEUDO != ord(target)
         ) + '\t{ 0, 0 }\n}'
 
 
@@ -335,13 +335,12 @@ class Keymap:
         # Construct raw mapping from source ASCII to target ASCII
         raw = {source: self.target[key.modifiers][key.keycode].ascii
                for source, key in self.source.inverse.items()}
-        # Eliminate any null mappings, mappings that attempt to remap
-        # the backspace key, or identity mappings
+        # Eliminate any identity mappings, or mappings that attempt to
+        # remap the backspace key
         table = {source: target for source, target in raw.items()
-                 if target
-                 and ord(source) != 0x7f
-                 and ord(target) != 0x7f
-                 and source != target}
+                 if source != target
+                 and source != BACKSPACE
+                 and target != BACKSPACE}
         # Recursively delete any mappings that would produce
         # unreachable alphanumerics (e.g. the "il" keymap, which maps
         # away the whole lower-case alphabet)
@@ -354,8 +353,8 @@ class Keymap:
         # Sanity check: ensure that all numerics are reachable using
         # the same shift state
         digits = '1234567890'
-        unshifted = ''.join(table.get(x, x) for x in '1234567890')
-        shifted = ''.join(table.get(x, x) for x in '!@#$%^&*()')
+        unshifted = ''.join(table.get(x) or x for x in '1234567890')
+        shifted = ''.join(table.get(x) or x for x in '!@#$%^&*()')
         if digits not in (shifted, unshifted):
             raise ValueError("Inconsistent numeric remapping %s / %s" %
                              (unshifted, shifted))
@@ -365,21 +364,22 @@ class Keymap:
     def altgr(self) -> KeymapKeys:
         """AltGr remapping table"""
         # Construct raw mapping from source ASCII to target ASCII
-        raw = {source: self.target.altgr[key.keycode].ascii
-               for source, key in self.source.inverse.items()
-               if key.modifiers == KeyModifiers.NONE}
+        raw = {
+            source:
+            self.target.get((key.modifiers | KeyModifiers.ALTGR),
+                            self.target[key.modifiers])[key.keycode].ascii
+            for source, key in self.source.inverse.items()
+        }
         # Identify printable keys that are unreachable via the basic map
         basic = self.basic
         unmapped = set(x for x in basic.keys()
                        if x.isascii() and x.isprintable())
         remapped = set(basic.values())
         unreachable = unmapped - remapped
-        # Eliminate any null mappings, mappings for unprintable
-        # characters, or mappings for characters that are reachable
-        # via the basic map
+        # Eliminate any mappings for unprintable characters, or
+        # mappings for characters that are reachable via the basic map
         table = {source: target for source, target in raw.items()
                  if source.isprintable()
-                 and target
                  and target in unreachable}
         # Check that all characters are now reachable
         unreachable -= set(table.values())